The nation's top real-estate appraisers have some words of wisdom for prospective sellers looking at headlines about stagnant or declining housing resale values and long-term prospects for high mortgage rates:

On the one hand, don't overreact to a market cycle that has probably reached its worst point -- and is likely to improve over the coming six to 12 months.

On the other hand, don't misinterpret the severe impacts on housing values that the current financing squeeze has produced. Don't assume your home is worth more today than it was a year ago; it may well be worth less on a "cash-equivalency" basis.

Those are the facts -- like them or not -- in the opinion of six nationally known appraisal experts located in different sections of the country.

Although they often work out of sight of home buyers appraisers play a key role in the U.S. housing market. Their judgments on resale values -- rendered for banks, savings and loans, and private owners --affect the size of loans lenders will make, as well as prices and terms demanded by sellers.

Appraisers insist that they don't actually set price levels in a community -- the "market" does that, they say -- but in fact they are often the professional arbiters of value in local housing markets.

Their opinions have taken on special significance during the past 18 months, as mortgage-financing costs have reached record levels.

Average housing-resale values have continued to creep upward in dollar terms during that period, but not necessarily in "real" terms, discounted for inflation. The national average price of a home sold last month, for example, was $68,000, up about 7 percent from the same month last year. Since inflation has been in the 10 percent range during that 12-month period, however, the true cost of the average-priced resale home has dropped by several percentage points in constant dollar terms.

On a strict "cash-equivalency" basis (what a house would sell for in cash at one time versus another), "There's no question that a $75,000 house in mid-1980 might be a $70,000 or even a $65,000 house today," says John L. Gadd, international vice president of the American Society of Appraisers.

A house in suburban Chicago that could be financed with a new, conventional 12 1/2 percent loan last summer "would almost certainly have sold for more" than it would sell for with today's 17 percent and 18 percent mortgage rates, according to Gadd, whose appraisal firm is located in Arlington Heights, Ill.

"That's an unpleasant reality," he notes. "After all, the American dream is that your home keeps going up and up in value. But that dream simply doesn't work when financing hits these levels."

Creative financing agreements -- seller take-backs of below-market second mortgages and deeds of trust, deferred payment balloon notes and other devices -- also have to mask true selling prices, according to Gadd. The cash-equivalent values of the financing concessions offered by the sellers have to be capitalized and then subtracted from the selling price to get true market value.

A town house might be worth $85,000 if the seller took back a $20,000, five-year second mortgage at 11 percent, in an 18 percent market, but might bring no more than $72,000 with no seller financing. The $13,000 difference is the capitalized value of the seller subsidy.

But should homeowners and prospective sellers be despondent about slipping values in soft markets, or a leveling out of values in stronger markets?

Absolutely not, says Dr. Byrl Boyce, a professor at the University of Connecticut's School of Business Administration and a nationally known lecturer on appraising.

Values are in a cyclical slump pattern. "Look back at 1973-1974 and prior credit crunches in many parts of the country and you'll see flattening out as well," he says "Yet the cyclical pattern eventually works itself out because the money market corrects itself. That's the historical cycle."

Boyce sits on the board of directors of a savings and loan association in suburban Manchester, Conn., and says, "We're looking at the likelihood of 13 percent to perhaps 14 percent rates later this year and into 1982" for planning purposes.

Boyce says that's the factoring-rate level that appraisers and homeowners ought to use in their own calculations if they wantto arrive at accurate market valuations for sales this fall and beyond.

The impending lower rates, in Boyce's view, ". . . mean that the deep financing discounts built into current market valuations are lower than necessary -- they're too harsh." Or in plainer terms, depressed or static housing values can be expected to move upward again later this year -- in real terms, not simply dollar terms.