This year, waiters face a tough customer: the IRS

It used to be that after a hard night of shuttling food from kitchen to customers, a waiter would take home his wad of tips and stash it. At income tax time, if honest, he'd declare all his tips and pay taxes on the sum. Many, however, would only report a bite-sized portion - and they usually got away with it.

Those days are gone. A new federal tip-reporting law that took effect April 1 puts teeth into income tax compliance. Restaurant owners now have to report a waiter whose declared tip income falls below a standard set by the new law.

Cooked up by Sens. Robert Dole (R) of Kansas and Charles E. Grassley (R) of Iowa, the law aims to net the Treasury about $5 billion by 1988. ''There is a huge underground economy, and Senator Dole wanted to narrow the gap,'' says an aide.

Waiters and restaurant owners are confused and angered by it.

Restaurant employees are organizing to get the law repealed. Taxpayers against Bureaucratic Unilateral (TABU), formed officially last month in Marysville, Calif. The group says it has 2,000 members in California and branches in South Dakota, Nevada, Wyoming, and Washington.

Members have sent Sen. Larry Pressler (R) of South Dakota ''three or four thousand letters,'' says an aide. TABU spokeswoman Martha Dennis says truck drivers in South Dakota have taken petitions to truck stops in other states. Senator Pressler and other congressmen have introduced seven bills to repeal the tax measure.

It's a complicated law. The National Restaurant Association is holding a series of 100 seminars across the country to explain it to restaurant operators. Waiters and bartenders must report their tips to the manager. If their total tips don't equal 8 percent of gross food and beverage receipts, the manager determines who's short. The shortfall is entered on the waiter's W-2 form - a ''red flag'' to the IRS for a possible audit.

Employees say the law is unfair. George Faust, a founder of TABU, says the measure covers some tipped employees and not others, such as hairdressers, taxi drivers, valets, and doormen.

Restaurant employees say that the new law assumes that their tips will equal at least 8 percent of their customers' food bills. While some big-city waiters and waitresses often make over 15 percent in tips, truckstop waitresses in the Midwest only pull in 3 to 4 percent, says Mrs. Dennis. She estimates her shortfall to be $300 so far this year and worries about an audit.

If a waiter has kept accurate tip records, an audit shouldn't be a problem, says one tax official. But Mr. Faust says, ''How can the individual prove he made under 8 percent, if the IRS can't? They've changed the burden of proof to being guilty until proven innocent.''

Restaurant owners whose waiters make very low tips can petition to be taxed at 5 percent, says a Senate Finance Committee spokesman. But Mrs. Dennis says some have been waiting since January for a reply, and others' appeals have been turned down.

Waiters say the new law ignores the seasonal, sporadic nature of the business. ''A tip is not a wage you can count on,'' says Fred Rosenbloom, a captain at Windows on the World restaurant in New York City. ''I think we should have to pay, but this way isn't equitable.''

The new law means adjustments for all restaurant employees. If they make a lot of money in tips, the tax on it can actually cancel out their hourly minimum wage of $2.01.

For example, a waiter who earns $330 for a 40-hour week ($250 in tips and a salary of about $80) would owe about $80 in taxes. He would receive no paycheck. If the tax is even greater than the wage, the operator issues a ''negative paycheck'' that the waiter can either pay back immediately or save up until the end of the year.

The law causes complications for the restaurant operators, too. Many are attending tax seminars to understand it. They are now required to submit to the IRS totals for food and beverages as well as charge-card totals and charged tips. ''It adds considerably to the workload,'' says Marsha Tarrh, general manager of St. Botolph Restaurant in Boston. Hiring bookkeepers and buying computers are two ways managers are adjusting.

''Its a very sensitive issue,'' says one manager. ''With negative paychecks it looks like people aren't getting paid.''