If you work for a furniture company and get on employes' discount on a new sofa, should that be taxed as income? How about free airline travel for stewardesses? Cut-rate parking for a secretary? Free lunches at the barracks for a state trooper?

Probably not, at least by most taxpayers' reactions. Almost without exceptions, workers seem to argue that each of these items is a valid fringe benefit - part of what an employee feels he's rightfully "due" as a perquisite of the job.

Well, the Internal Revenue Service disagrees, and the dispute has set off a major flap over taxation of fringe benefits that is producing a serious tug-of-war between the IRS and Congress. And so far, Congress seems to have the longer end of the rope.

The question is a thorny one. According to the federal income-tax code, Americans are supposed to pay taxes on all kinds of income - cash or indirect benefits - unless they're specifically exempted by statute. The IRS then is supposed to set guidelines for the rest.

So far, most fringe benefits have gone untaxed by custom, if not always by statute. Depending on the occasion, policymakers either have chosen intentionally to overlook them, or else simply have thought they weren't worth the effort to collect.

But last summer, Jerome Kurtz, the new Carter-appointed IRS commissioner, served notice the agency planned to publish new regulations that would require taxpayers to pay taxes on most fringes - from free airline tickets to subsidized meals in company cafeterias.

In the process, the list of items mentioned has included suggestions for taxing virtually every fringe benefit or perk that accrues to an employe - even the value of a sandwich on a company-sponsored picnic. Predictably, the storm has been raging ever since.

Most tax analysts agree Kurtz more than has a point. Even if the perks are part of the job, stewardesses who qualify for free airline tickets, for example, really are receiving a form of income that isn't available to other workers.

So are university professors whose kids receive free tuition. (The professors argue their salaries are so low that without the free tuition, they wouldn't be able to afford to send their youngsters to school, but that isn't the issue. The question is, does the benefit qualify as income?)

And the courts recently have ruled that lunches served free to state troopers qualify as income, even though such meals traditionally have been tax-free. (The late-coming decision has left some troopers with big back tax bills.)

The difficulty is, the argument involving fringe benefits isn't all that simple. In the first place, present law already taxes some fringe benefits, while exempting others, so any new expansion is bound to exacerbate existing inconsistencies.

For example, company payments for employes' medical insurance or group life insurance under $50,000 are specifically exempt under present laws. So are pension fund contributions. Services of a chauffeur are exempt when the employe is on company business.

For another thing, some of the proposals for taxing fringes clearly have been overblown. The suggestion to require employes to pay taxes on food they eat on a company-sponsored picnic, for example, seems patently absurd to all but a few purists.

Other such proposals are in a gray area. Cut-rate parking provided in a mid-city lot may genuinely be counted as income, but what about parking on a factory lot outside of town? Where else could employes go to park? In a nearby meadow?

Then there's the question of how far to go in taxing presently exempt benefits. Isn't the free medical care physicians provide each other as a so-called "professional courtesy" actually a form of income? What about the free housing many churches provide their clergymen?

The Carter administration has attempted to portray the present treatment of fringe benefits as a break for executives and upper-income taxpayers - some sort of perquisite for the rich and powerful, akin to the president's campaign against the three-martini lunch.

As in the case of the business lunch, however, the rhetoric may have produced a backlash. In fact, taxpayers in virtually all middle-income brackets enjoy some sort of fringe benefits. And many are bent on maintaining them - and keeping them tax-free.

Predictably, Congress has not responded harshly to the IRS initiative. Last year, in an informal order, the House Ways and Means Committee forced IRS to agree not to issue final regulations on the fringe issue until July 1. Just this past month, the committee put it off further, to 1980.

The battle won't be an easy one. Apart from the issue of voter opposition, many companies argue convincingly that some fringes help spur employee productivity, or at the least boost workers' morale. Still, not all perks are in that category. Some shifting clearly is needed.

What some analysts are hoping for is a more serious examination of the fringe benefits question after the current flap dies down - possibly focusing less on the flashly "equity" elements of the issue and more on how much income actually is involved.

Tax experts say in some cases - factory parking-lot spaces and cut-rate company cafeteria prices, for example - an order to tax fringe benefits would be difficult to enforce and hardly worth the bother involved. In others, such as the free tuition for professors' youngsters - it may be worth the stand.

In any event, there's little real doubt that the issue of whether to tax fringe benefits will be among the more controversial facing Congress in coming months. The question is, whether it all can be resolved rationally on both sides.