Investors wrestling the wartime financial markets of early 1991 should keep two considerations firmly in mind: It usually makes sense to hold on to stock or stock-fund investments in volatile markets, whether up or down.

Most folks turn profits by holding patiently for five years or more, not by trying to make fancy moves amid a maelstrom. And if you do sell or buy in 1991, you'll find it a more costly proposition than in the past. That's because a number of big brokerage firms have boosted their commission rates by about 5 percent, and others are considering doing the same.Adding to investor costs even more, many firms are boosting postage-and-handling charges, with Merrill Lynch & Co. soon to be highest in this regard at $4.85 per transaction. As a result, it's never been more important to compare commission charges carefully and to be sure you're receiving the service you deserve for the price you pay. Some brokers are more than worth their keep, while others aren't, the same as with any profession.

"With the sharp decrease in purchases of securities, the brokerage firms found they couldn't spread the costs out on a unit basis," said Jeffrey Schaefer, senior vice president of the Securities Industry Association, the trade association for the brokerage industry. "They are simply passing on the cost increases they face."

Full-service firms don't seem to be concerned that raising commissions may scare off clients.

"Our clients are more interested in the highest possible quality of service and the financial strength of the provider than they are in the commissions or fees," said Fred Yager, a spokesman for Merrill Lynch, which enacts its 5 percent commission hike for any trade over $5,000 on Feb. 1. "The increase is necessary to maintain that quality."

There are, of course, discount brokerage firms that feature lower fees than full-service companies but don't offer any advice. However, price-conscious investors would do well to compare among them carefully.

"While the three major discount brokers, Fidelity Investments, Charles Schwab & Co. and Quick & Reilly, are cheaper than full-service brokers, they are quite expensive relative to the cheapest of the discounters," observed John Markese, director of research for the 100,000-member American Association of Individual Investors.

For example, purchasing 100 shares of a $30 stock at a full-service brokerage firm incurs a commission charge of $83.69 at Dean Witter Reynolds Inc.; $85.50 at Merrill Lynch; $91 at Smith Barney, Harris Upham & Co.; and $86 at PaineWebber Inc. That same transaction at one of the major discount brokerage firms is $49 at Schwab; $49 at Quick & Reilly; and $47 at Fidelity.

Moving to deep-discount firms, that identical trade is a much-lower $31 at First National Brokerage; $36 at Muriel Siebert; $27 at Pacific Brokerage; $22.99 at K. Aufhauser; and $35.50 at St. Louis Discount.

Thus far, discounters don't seem to have any commission boosts on tap.

"The increase in postage cost is having an impact on anyone who does a lot of mailing, so we are looking at steps to be more efficient, such as using a thinner paper stock," said Hugo Quackenbush, senior vice president at Charles Schwab & Co. "As of now, we don't have any plans to increase commissions, add a surcharge for trading or add an account maintenance fee."

In regard to stock funds, there have also been increases in charges worthy of note if you're considering a purchase or sale. It's true that a lot of traditional "load" (initial sales charge) funds, under pressure from competition, have reduced their 8.5 percent loads to 5.75 percent or 4.5 percent over the past few years. However, many traditional "no-load" fund families have been adding 2 percent to 3 percent sales charges.

"Furthermore, many funds have initiated "back-end" fees for selling shares, with perhaps a 5 percent load the first year you own your shares, declining a percentage each year, so it becomes important to carefully monitor and compare," said Don Phillips, editor of the Mutual Fund Values investment advisory service.