Postmaster General William F. Bolger said yesterday that if the independent Postal Rate Commission rejects the U.S. Postal Service's request for a 5-cent increase in first-class postal rates, the commission should be abolished.

The rate commission is expected to decide next week whether to increase the postal service's rates for different classes of mail -- including 20 cents for a first class stamp -- or accept the proposal by the rate commission's Office of the Commission of 18 cents for first-class mail and higher second- and third-class rates. The final decision, however, will be made by the postal board of governors.

Bolger, using the nation's economic climate as a backdrop, also warned the postal service's unions against seeking high wages and benefits or work slowdowns during contract negotiations this spring.

"In this year of critical economic importance, this nation cannot tolerate a postal work stoppage of any kind," Bolger told the National Press Club. "Nor can I. And if anyone should foolishly pursue such a course, I will not hesitate to" exercise available legal options.

The Postal Service, in addition to financial problems of its own making, is slated to receive from the Reagan administration a substantial reduction -- from $789 million in 1981 to $500 million in 1982 -- in outlays to absorb the postal service's losses because of lower mailing rates charged to nonprofit organizations.

Bolger said that he hasn't seen proposals to cut the postal service's funds but he would "probably give very supportive considerations" to budget-slashing recommendations.

A budget document prepared by the Office of Management and Budget also recommends elimination of Saturday mail service. But, Bolger said, even with budget cuts he has no plans to eliminate Saturday mail delivery or any other services. Instead, he said, he would try to improve productivity.