Washington postal officials yesterday said that hundreds of federal income tax returns will have to be mailed back to their senders because those taxpayers failed to use enough stamps on their IRS forms.

"They should have had it weighed before they mailed it," sighed George Conrad, spokesman for the main Washington post office that by noon yesterday had received nearly 700 tax forms without enough postage. The standard 20-cent stamp would be enough for a simple short form, he said, but might not be enough for more complicated returns, which may cost as much as 37 cents to mail.

IRS spokesmen said they will have no sympathy. "If it's not here on time it's a late return," said IRS spokesman Wilson Fadely. If the affected taxpayers owe the IRS money "they will be subject to penalties."

Washingtonians yesterday were practicing the annual spring rite of procrastination and then raced the IRS office, banks, accountants and ultimately, the post office to mail returns by the witching hour.

D.C. government officials, meanwhile, held a rain-soaked rally on the U.S. Capitol steps to protest "taxation without representation" of District residents who do not have voting congressmen or senators. Cheerleaders from Woodson and Coolidge High Schools led the cheers. Across the city, Libertarian Party members held their own protests at area post offices, objecting to any income tax at all and calling it theft.

Last minute filers who failed to use enough postage on their forms, may be lucky enough to get their returns back in a few days, but only if they remembered to write their return addresses on the outside of the envelopes, postal officials said.

"If it does not have a return address, then it is sent to the dead letter office in Philadelphia where they are authorized to open non-deliverable mail," Conrad said. He said he didn't know how long it would take officials to return the forms to the senders. "It depends on how much of a backlog they have. It could take a week or more."

Postal officials are unsure whether this year's number of returned forms is unsual. "We've never been asked before," Conrad said.

Postal officials in Northern Virginia and suburban Maryland said they didn't think they had a large number of tax returns without sufficient postage. Any envelope weighing more than one ounce would cost an extra 17 cents for each successive ounce.

Fadely said that there are various late filing penalties, some of which range from five to 25 percent per month, but he added that "if it's only a few days late we would not be talking about a whole lot of liability."

Last-minute filers came in all forms. President Reagan's taxes, released yesterday, were dated April 14, while an intern had to be dispatched from Vice President Bush's office with a laundry list of forms to be picked up for office staffers at the local IRS office on E Street.

Some people even enjoyed the rush. "Isn't it comical," said one smiling young woman joining the line for information at the IRS office. "All the lines, all the reporters . . . . It's fun."

On a subway car carrying several acknowledged last-minute filers, one young man said he mailed his taxes at the downtown post office every year just so he could watch all the cars and people making a run for it. But most said they filed late due to procrastination or because they owed money and wanted to hold onto it as long as possible.

"In my case it's laziness and stupidity," said systems analyst Mike O'Brien as he picked up a form yesterday. "This year they owe me a refund."