The Washington Consumers Checkbook magazine will rate private firms here next month that are charging federal workers from $11 to $35 for computerized advice on pension planning. Checkbook is well known to Washington area federal workers for its annual ratings of health insurance plans.

Some employes can boost their retirement benefits as much as $5,000 per year, the magazine says, if they switch from the old pension plan to the new retirement program. But there are many federal workers, especially long-time workers, who would be better off not to switch. The trick is knowing which group you fall into, and that is where the special computer-planning services are willing to help -- for a fee.

Federal workers hired before 1984 have until Dec. 31 to decide whether to remain in the old Civil Service Retirement System (CSRS) or switch to the new Federal Employees Retirement System (FERS). FERS covers most people hired since 1984 and offers benefits based on Social Security, a modified federal pension and earnings from optional investments in a tax-deferred thrift savings plan.

CSRS bases benefits on salary and length of service and offers participants a more limited thrift investment option.

Although the pension open season started in July, less than 2 percent of the 2 million people who must make a choice have moved into the FERS plan.

Several Washington area firms are offering computerized benefits profiles for fees ranging from $12 to $32. Those printouts show what workers could expect under each plan.

Next month Checkbook (347-9612) will outline the differences in the two plans and tell workers what they get, and don't get, when they pay for outside help. The consumer magazine generally gives high marks to the computerized advice offered by the Office of Personnel Management. It also gives examples of how some of the computer-advice services come up with different data based on assumptions they've built into their programs.

The one-time, one-way pension choice -- you can only switch from CSRS into FERS, and once in, you cannot return to the old system -- is a major career decision for government workers regardless of age, salary or length of service.

Many employes are apparently holding back because they want to see what changes Congress may make in the FERS program that would affect their future Social Security benefits and survivor benefits under Social Security.

Firms offering the computer benefits printouts include:Chambers Associates, $25. Telephone, 857-0670. PIPS, $32. Telephone 532-1631. SORT, $32. Telephone, 354-8337. GRB Inc., $25. Telephone, 461-9100. American Financial Services, $12. Telephone, 451-1300.Job Mart

The General Services Administration has openings in Crystal City for clerk-typists, Grades 2 through 4 and a secretary-typing, GS 3 through 6. Call 557-8563.

GSA in New York City is looking for a GM (merit pay) 13 supervisory safety engineer and will pay relocation expenses. Call (212) 264-8295.