The D.C. Public Service Commission is investigating billing practices at both the Washington Gas Light Co. and Potomac Electric Power Co. to check allegations that some customers are treated more equally than others.

Chief PSC accountant Seymour Manheimer said his office was concerned with "a morality question" raised by a special VIP list of customers at Pepco. A similar list exists at Washington Gas, he said, and added that the probe would also touch on the gas company's method of estimating bills.

The Pepco special list includes members of Congress, judges, elected officials and other "prominent area people" as well as schools, hospitals, fire stations and government buildings. All such customers receive telephone calls on overdue bills rather than the stiff computerized letters that ordinary customers are sent.

The Washington Gas VIP list has between 1,000 and 1,500 names out of its 530,000 metropolitan area customers, all individuals, according to media relations manager Paul D. Young. "All customers get a written notice to begin with, but after a couple of them, a telephone contract is made to those on the VIP list." Those not on that list continue to get computerletters asking for payment.

"It may be more important to get the people informed about their problem in the Cardozo area than the VIPs," Manheimer said, referring to a run-down area north of Shaw in the District of Columbia. "There's a morality question involved."

He said the Public Service Commission had received a number of complaints that the Washington Gas practice of reading meters every other month had led to unfair billing. Customers who may have been away and used no gas were later dunned for the bill, he said, while other meters have never actually been read at all.

"We have to determine whether the cost of monthly readings for everyone would be justified by the result," Manheimer said.

The commission will try to determine whether one meter reader could do the job for both Pepco and the gas company, he said. The study, which involves only the 153,000 D.C customers, is expected to be completed by the end of November and Manheimer said he would submit recommendations to the commission at that time.