IN KINDER TIMES, there would be no question but that the United States would decide without much fuss to keep in force the terms of an old, admittedly imperfect arms control agreement while working on a new and supposedly better one. For the Reagan administration, however, it is a tough question whether to continue respecting the never ratified SALT II treaty, which the president at first pronounced "fatally flawed" but later reluctantly agreed not to undercut so long as Moscow didn't undercut it either. The date at which the treaty would have expired is coming up, and even sooner the United States must decide whether to retire some old missiles in order to make room for new ones under the SALT II lid.

Within the administration, a range of opinion is evident. Important figures on the political side continue to regard SALT II as a symbol of the failure of past arms control accords to do more to strengthen American security and as a damaging restraint on American arms-building programs. That the Soviets say they can still live with SALT II is taken as evidence that the restraints aren't tough enough. It is suggested that breaking out of SALT II, far from complicating the talks in Geneva, will give Moscow added incentive to consider the proposals Washington is making there. Anyway, why should the United States honor an agreement when many questions about Soviet compliance are still hanging?

Elsewhere in the administration, including on its military side, there is another emphasis. There is, for instance, a disposition to believe that the Soviets are considerably better placed to deploy threatening new weapons than is the United States without SALT II; to understand why, you need merely look at the bedraggled history of the MX. Military officials can identify no good military reason to go past the SALT limits. Diplomats worry how American allies would react to the spectacle of an American breakout. Many observers feel that Congress would react sharply to that spectacle by taking new budget hostages. The best cure to the flaws in past restraints and verification stand- ards, many of these officials feel, is progress in Geneva -- something arguably more likely to come in an atmosphere undisturbed by the detonation of SALT II.

Liberal arms controllers argue that the very procportant as the product -- the agreements, which so far have turned out to be pretty thin. At this late date, however, no one can really think that President Reagan is going to change spots and adopt this sort of questionable reasoning. Nor is it necessary for him to do so. In his own administration, among people devoted to his conservative principles, there is comfortable support for continuing to observe the terms of SALT II. He should do it.