STANDARD advice for tourists going abroad is to have their foreign-made cameras and other photographic equipment registered before they leave home. It's hard to say how many travelers take that advice, but, says the U.S. Customs Service, it could save them homebound hassles, especially if they are returning from the country where their camera is made.

If you have toured West Germany and show up at John F. Kennedy Airport toting an unblemished Leica, you could run into delays trying to prove prior ownership of the camera to a questioning Customs official on the lookout for anyone trying to avoid duty fees.

Chances are you will be able to convince the inspector that the camera is yours, perhaps simply by naming the U.S. store where you purchased it. Or you may look like a tourist, who naturally would be carrying a camera. But if your story is doubted, your camera could be held until you produced acceptable verification of purchase. Or you could be assessed duty, for which you would later have to seek a refund. In any case, you have lost precious minutes in the rush to catch a connecting flight.

The advice holds true for foreign-made electronics goods, watches and binoculars as well as jewelry, high-fashion clothing, fur coats and other valuables you take on an overseas trip.

Customs will register any foreign-made item that bears a serial number, providing the traveler with a validated form that should be carried with you whenever you take the camera or other items abroad. Frequent travelers tuck the form into their camera case to have it handy whenever needed.

Items can be registered at any Customs office. Locally, the Customs office at 1301 Constitution Ave. NW (Room 4128) is open Monday through Friday from 8:30 a.m. to 5 p.m. At Dulles International Airport (at the Customs office in the Gateway Building near the terminal) and Baltimore/Washington International Airport (international arrivals area), the hours are 8 a.m. to 5 p.m. The process takes about five minutes, and you must have the item with you.

For items with no serial number, such as valuable jewelry, Customs suggests carrying such proof of ownership as a sales receipt, a jeweler's appraisal or insurance documents. If it clears you through Customs more quickly, it's worth the effort. FOREIGN FAVORITES: America's national parks are popular destinations for visitors from abroad. Their favorites, reports a National Park Service survey, are (in descending order of visits):

* Grand Canyon National Park in Arizona

* The national park areas of the Nation's Capital

* Everglades National Park in Florida

* Yellowstone National Park in Wyoming

* Zion National Park in Utah (an easy half-day's drive from the North Rim of the Grand Canyon)

With the exception of Mexicans and Canadians, who generally were not included in the study, the largest percentage of park visitors came from West Germany, Switzerland and the United Kingdom. About 60 percent of them got to a park in a rented car, and a fifth of them spent at least one night camping out.

Surprisingly, the visitors' biggest expense in 1982, when the figures were compiled, was not food or lodging but "gifts and souvenirs," which leads one to suspect a lot of European youngsters are sporting cowboy hats or Indian feathers.

RODEO EAST: Eastern cowboys don't have to head for the Southwest to watch an authentic rodeo. Every summer for almost 50 years the broncs and bulls have been throwing riders in, of all places, a corral in southern New Jersey.

The Cowtown Rodeo, held outside the tiny community of Sharptown, N.J., at the Cowtown livestock auctioneers, is a regular Saturday night event from about Memorial Day through (this season) Sept. 10. It all started during the Depression as a way to attract people to the Salem County Fair, and now draws weekly crowds of up to 4,000.

Each Saturday night's program, which begins at 7:30 p.m., offers seven events: team roping, calf roping, steer wrestling, bareback riding, saddle bronc riding, bull riding and women's barrel racing (around a cloverleaf pattern of barrels). Winners earn money and points in the Northeast Circuit of the Professional Rodeo Cowboys Association, which sanctions the Cowtown Rodeo.

Sharptown is eight miles east of the Delaware Memorial Bridge on U.S. 40 just off the New Jersey Turnpike. Admission is $4 for adults and $2 for children under 12.

Participants usually are weekenders for whom riding and roping are recreational sports. But most compete regularly on a circuit that also includes a number of county fair rodeos. The Northeast Circuit finals, Sept. 9-11 at Sharptown, is expected to draw the region's top 15 cowboys.

For more information: New Jersey Division of Travel and Tourism, CN 826, Trenton, N.J. 08625 (609) 292-2470 or Cowtown Rodeo (609) 769-3200.

COUNTRY GETAWAY: Off in the Blue Ridge foothills of Virginia, about 75 miles from Washington, stands a three-story hand-hewn log house built in 1794, during George Washington's second term as president. Across the road is a small cabin, believed to be one of the oldest residences in Rappahannock County. Both are set on 125 acres of forested hills and pastureland down a mile-long gravel road. A pathway leads to a small fishing pond.

Now fully restored as Bowyer's Mountain Hill Farm, the main house and cabin offer a quiet, secluded weekend retreat for up to four couples, including lodging and a complimentary country breakfast. The farm is near Castleton, about eight miles from Sperryville and 15 from Little Washington, where the Inn at Little Washington is a popular dining spot.

The farm, named after it's original owner (but don't pronouce the "w" in Bowyer's), is the project of Harry I. Clarkson, a Washington advertising and marketing executive, who bought it in 1979 as "a place to recharge the batteries on weekends." After substantial renovation, he opened it to guests this summer.

Accommodations in the log house include the Red and Green rooms (with shared bath), named for the decor, and the Stone Room (private bath), which was once the kitchen. When the house was first built, the Stone Room was situated over a spring, providing 18th-century running water. The one-room cabin (with private bath) also sleeps two. The cost is $75 to $99 per night for a couple, with a reduction for guests staying two nights.

Diversions include hiking, birdwatching, riding (available from nearby stables), bass and perch fishing in the stocked pond and antique-browsing in surrounding communities. But above all, says Clarkson, it's a place to relax.

For information: Bowyer's Mountain Hill Farm, P.O. Box 100, Castleton, Va. 22716. For reservations, phone 363-6900 weekdays or (703) 987-8222 weekends.