"Dancing Between the Raindrops" was in step Saturday night at the 1979 dinner-dance preview of the 1979 Decorators' Show House. The September storm pounded down, but inside the circus-size tent only a dribble here and there leaked through. There were no dampened spirits, certainly not those of Muriel Threlfall, the party chairman, who counted 450-odd $60-ticket holders attending despite the weather.

The party was doubtless the biggest held at Oxon Hill Manor since then-under secretary of State Sumner Welles entertained President Franklin D. Roosevelt and other '40s figures there.

The star of the evening was Mstislav Rostropovich, maestro of the National Symphony, the beneficiary of the fund-raiser; During the cocktail-hour conversation, Rostropovich deplored the Soviet government's early closure of the Renwick Gallery's 19th-century Russian art show. Rostropovich said he had also heard that the Soviets will cancel the scheduled tour of the Moscow State Orchestra. "It isn't officially announced yet," he said. "But I hear it will happen."

The Decorators' Show House is a project of the Women's Committee, of the National Symphony. Marcelle Cahill, chairman of the Show House, said the house redecorated by 27 Washington area interior designers, will be open at 6901 Oxon Hill Road, Oxon Hill, Md., through Oct. 22.