The Supreme Court ruled 9 to 0 yesterday that lunch allowances paid 15 years ago to employes on daytime out-of-town trips weren't wages subject to federal withholding taxes.

The significance of the ruling is murky, partly because in 1969 the International Revenue Service put employers on notice - but did not issue a formal ruling - that it considered such allowances to be wages from which taxes must be withheld.But as of APril, 1977, the IRS had before it 295 challenges to its position.

An IRS spokesman said yesterday, "It's difficult to see what the ramifications are. It's an area we need to take a look at . . ."

The decision involved the Central Illinois Public Service Co., which in 1963 reimbursed employes up to $1.40 per lunch consumed during non-overnight travel. "In 1963 not one regulation or ruling required withholding," Justice Harry A. Blackmun emphasized in the opinion for the court.

Not until 1967 did employers get the first hint, from the Supreme Court ruling, that the lunch allowances might be wages.

Not until 1971, after an audit, did the IRS tell the utility that it should have withheld and therefore owed $25,189 plus $11,427 in interest. Central Illinois paid but filed a court challenge.

Until the 7th U.S. Circuit Court of Appeals upheld the IRS in 1976, "no court had ever held lunch reimbursements to be wages for withholding purposes," Blackmun said in an opinion for the court.

He said the court was not deciding the validity of a possible future IRS regulation that lunch reimbursements require withholding.

In separate opinions, three of the justices said that the IRS abused its discretion in making the utility retroactively liable. Justice Potter Stewart joined only in the result of the decision.

In another case, the justice ruled 8 to 1 that the maximum 20-year sentence for bank robbery can be enhanced to 25 years if a firearm is used, but can't be raised again with a conviction on a seperate charge of using a gun in "any felony."

The ruling involved MIchael L. and Tommy W. Simpson . They were convicted of two bank robberies, with handguns, in Middlebore, Ky. Each drew 25 years on seperate firearm charges. Because the sentences were consecutive, each faced 70 years in prison. They now face 50.

Justice William J. Brennan Jr. wrote the opinion for the court. Justice William H. Rehnquist disented.