Commissioner Roscoe L. Egger Jr. heads into 1986 with the distinction of staying on the job longer than any IRS chief since World War II. Egger started work on March 14, 1981, almost five years ago. Many of his predecessors served two or three years. Egger has lasted longer than any commissioner since Guy Helevering was the nation's chief tax collector from 1933 to 1944. There is no public sign that Egger, who is on vacation until next week, is thinking of leaving.

Egger has headed the IRS during one of its rockiest periods. The 1985 filing season was widely recognized as having been a disaster, with millions of returns lost in the system, the time required to process refunds running far worse than average, computer foul-ups widespread and occasional incidents of tampering or destruction of returns.

The service has taken several steps to see that the 1986 filing season goes better. It has increased computer-hardware capacity by 25 to 50 percent at its 10 service centers. The number of central computer units nationwide will rise from 48 last year to 68 in 1986.

At the Philadelphia center, one of the most troubled locations, where tax returns from the District and Maryland are processed, two new computers have been added to the center's four central processing units. Two computers also were added to the center at Memphis, where returns from Virginia are sent, and where the last filing season went more smoothly than in Philadelphia, bringing the total to six there as well.

According to Daniel Capozzoli, deputy assistant commissioner for computer services, the computer program for handling tax returns also has been upgraded, so that returns can be processed 68 percent faster than they were at the beginning of last year. And the IRS has replaced its ad hoc on-the-job training for certain job categories with formal programs. The computers in each center will be monitored by more people, and managers will be on duty 24 hours a day, Capozzoli said. Partnership Shelters Grow

Recent IRS statistics indicate that partnerships are increasingly used as a method to avoid taxes. From 1978 to 1982, the number of partnerships grew by 23 percent, from 1.2 million to 1.5 million. The growth occurred principally in the oil and real estate areas. Unlike corporations, partnerships can pass through losses to their individual owners. When Congress passed the 1981 tax law, which expanded depreciation and other business deductions, the amount of expenses that could be claimed as losses for tax purposes increased, thus enhancing the tax-shelter potential of partnerships, the IRS study found.

Another sign that partnerships are being used to reduce tax exposure is that the oil and real estate partnerships reported larger and larger business losses during the period studied, yet the number of returns filed increased. In ordinary circumstances, industries that lose money tend to contract. Rules on Fringe Benefits

While the House, as part of its tax-overhaul bill, has just voted not to tax many employer-provided fringe benefits, the IRS has just issued new regulations detailing how fringe benefits should be taxed. The proposed rules, part of the 1984 tax law, would not tax such common benefits as health-insurance premiums paid by employers or other benefits the House specifically declined to tax in the overhaul bill now in the Senate.

Instead, the 200 pages of proposed rules spell out how to value for purposes of taxation personal trips in a company car or airplane and when employer-provided meals qualify as taxable income to an employe, among other changes.

The regulations also ask for public comments on how "frequent-flyer" free flights on airlines should be taxed. The IRS believes that such flights constitute taxable income if they are used for personal travel but were "earned" during business flights paid for by the employer.