Should the United States indict former Philippine President Ferdinand E. Marcos and his wife, Imelda, for conspiracy and fraud?

That question is on President Reagan's mind this week as he reviews an official briefing on the subject, and the prospect of going after the former dictator in the courts must be tempting.After all, it seems clear that the Marcoses bilked not only their own countrymen but also tried to cheat Americans. As The Los Angeles Times puts it: "Prosecutors uncovered an elaborate paper trail making clear that the Marcoses used millions of dollars in Philippine government funds to purchase New York City real estate and then schemed to conceal their ownership and to evade U.S. taxes."

Besides showing that no one is above the law, indicting the Marcoses would please current Philippine President Corazon Aquino just when a favor from Washington might be in order. The Aquino government, which wants the former dictator indicted, recently backed off from its demands for a huge increase in compensation for the U.S. military bases in the Philippines.

As attractive as such considerations are, the case for not indicting Marcos is stronger.

To begin with, it would be hard to indict Marcos without breaking the American government's word to him. Marcos was induced to leave the Philippines for Guam and then Hawaii only after being assured of a safe haven in exile. That assurance clearly implies freedom from federal prosecution, though not from the civil suits that the Philippines already have filed against Marcos.

Keep in mind that by inducing Marcos to leave without a fight, Washington avoided a bloodbath in the Philippines and helped a government friendly to the U.S. to come to power in Manila.

Keep in mind, too, that if the U.S. government starts prosecuting Marcos, it will become harder for Washington to use the prospect of a safe haven in exile to lure other dictators out of power.

The decision on the Marcoses should be made on the basis of what best serves America's own interests. Those interests would seem to be most effectively furthered by forgoing the proposed indictment.