A rider to the House version of the tax bill under consideration last night by members of the House-Senate conference committee would effectively exempt the Internal Revenue Service from Freedom of Information Act requests which provide the only outside checks on the agency, a prominent tax researcher and IRS critic said yesterday.

But the IRS, which requested the bill's introduction, and some House staffers contend that the measure is essential to prevent the release of date they say would be a "how-to" guide to cheating on taxes.

Congressional staffers yesterday expected the conferees to act on the rider as early as late last night.

At stake is a six-year-old suit by statistician Susan Long and her husband Phillip to obtain data tapes from the service's Taxpayer Compliance Measurement Program (TCMP). The program, which analyzes tax returns since the year 1963, helps the IRS select which types of taxpayers are most likely to underpay their taxes.

One alternative before the conferees, offered by the Long's home state senator Henry Jackson (D-Wash), would exempt the Long's suit from the rider if they agreed not to claim the date before Jan. 1, 1983. A Jackson aide said yesterday that the measure would give Congress time to analyze the tapes to assess the IRS claims.

Susan Long claims that introducing the bill as a rider was an attempt to railroad the measure through Congress. "The problem is that this thing gets buried in the tax bill," she said last week. "It should get a fair hearing, as a minimum."

With the case slated for Supreme Court consideration as early as October, IRS lawyers see the bill as necessary, to block the possibility that the Supreme Court will release the tapes. Their fears may stem from the fact that the Longs have a 12-0 record against the IRS in freedom of information cases.

Susan Long, who wants the tapes for her research, and her lawyers say the legislation could allow the IRS to prohibit disclosure of tax information even to congressional committees and other government agencies. They have received support from economists such as Joseph Peckman of the Brookings Institution, who worry that the rider would stifle academic inquiry.

A staffer on the oversight subcommittee of Ways and Means replied, "That's absolutely wrong." The rider, he claimed, would not affect such disclosures because they are guaranteed by other sections of the FOIA. But Judith Bendich, a Seattle lawyer representing the Longs, asserted yesterday that the rider includes language that would invalidate those protective sections.

The House bill would prohibit disclosure of the standards the IRS uses to select which returns will be among the more than 2 million audited annually, or if data used to set those standards. Rather than letting the courts decide whether it is safe to release such information, the bill would give the secretary of the Treasury the power to decide.

A similar bill was introduced in the Senate by Finance Committee Chairman Robert Dole (R-Kan.). But after urgings from Washington senators Jackson and Slade Gorton (R-Wash.), and agreed to put off action on the measure pending hearings in September.

Those on the side of the IRS say any delay would be too long. "The fear is that Congress won't be able to act before October," when the Supreme Court returns from its summer recess, said the oversight subcommittee staffer. "If these tapes did come out, it might indeed be very damaging to the IRS tax collection efforts," he said.

A San Francisco circuit court of appeals ordered the tapes released early this year, but they remain with the IRS, pending a decision by the Supreme Court.