Talleyrand's famous cautionary note to a young French diplomat (''Surtout, pas trop de zele,'' or words to that effect) cuts two ways as we approach next week's summit. Roughly translated, it means: ''For Heaven's sake, don't go overboard.'' That's good advice for the U.S. side: The summit air is thin; euphoria can set in quickly; Gorbachev is entitled to very little benefit of doubt.

And it's good advice for America's super-conservative commie-bashers as well. If there is a danger in trying too hard for accommodation on arms control, human rights or resolution of ''regional conflicts,'' there is an equal danger of missed opportunities. Just possibly there are things Gorbachev might be doing in his own interests that might be in our interests as well.

The point is not that we should buy the much-trumpeted Soviet ''new thinking'' on its face. The point is that we should also be wary of anti-communist zealots who, it becomes apparent, will stoop to almost anything in efforts to prove that absolutely nothing has changed.

A perfect illustration is to be found in a recent op-ed page piece in the Washington Post from the polemic pen of Edward N. Luttwak, a prominent right-wing scholar at the Center for Strategic and International Studies here.

Lenin lives! That's Luttwak's thesis. Nicaragua is his proving ground. ''Americans are still furiously debating the nature of the Sandinista regime in Nicaragua and its intent in regard to the {Guatemala City} peace plan,'' he writes, ''but surely that question has been settled conclusively by the photos that appeared on Nov. 3, the day after the opening of the Party Congress in Moscow. In them, Daniel Ortega of Nicaragua can be seen sitting next to Erich Honecker of East Germany and Gen. Wojciech Jaruzelski of Poland, in the section reserved for leaders of Leninist governments in good standing.''

Planting himself firmly on this evidence -- and nothing more -- Luttwak leaps to the conclusion that the Sandinistas have been admitted to the ''very exclusive club of governments that the Soviet Union regards as permanent, organic allies.'' Seating arrangements, he says, serve as ''very precise political indicators.'' By clearly distinguishing Ortega from your garden-variety Third World Socialists, the Soviets have announced an irreversible commitment to a ''fully fledged Leninist regime.''

This means Soviet acceptance of an open-ended burden of economic and military aid; there can be no ''back-sliding.'' This, in turn, signifies Ortega cannot be expected to make any concessions under the terms of the Guatemala peace plan (cease-fires, a certain relaxation of repression in Nicaragua and all the rest) that would pose the slightest challenge to the Sandinistas' monopoly of power.

Luttwak is sure of it. ''Am I the only person who has seen'' the telltale photographs, he asks smugly. Even a rhetorical question invites an answer: No, Edward, you are not the only one; but you may be the only one who failed to notice that the photographic display in The New York Times actually consisted of two photographs, one of Honecker and Jaruzelski seated side by side, and another of Ortega alone. There was white space between the two shots and nothing in the caption to suggest that the three men were even in the same room.

Nor was there, according to authorities I've talked to, a special section ''reserved'' for ''Leninist governments.'' True, the communist countries of Eastern Europe were lumped together. But Ortega was no closer to them than he was to a mixed gaggle of Socialist leaders, including representatives of India's ruling National Congress, the Italian Communist Party, and the PLO's Yasser Arafat. So much for the ''political precision'' of Soviet seating arrangements or the claim that ''the nature'' of the Sandinistas, or their intentions, has been ''settled conclusively.''

There is, on the contrary, still room for a competing theory of the case: that the Soviets actually are fed up with bailing out Nicaragua's ''basket-case'' economy and bank-rolling the Sandinistas' struggle against the contras -- and that they told Ortega as much in Moscow. That he got some message is evidenced by his sudden flip-flop, right after his return, on his willingness to deal, at least indirectly, with the contras and by his various moves, however inclusive, to comply with commands of the Guatemala Plan. By this theory, Ortega is under Soviet pressure to do everything he can, short of a wholehearted embrace of democracy, to get the contras off his back.

It would be unwise to lean too heavily on this theory -- or any theory of how the Central American peace process will turn out. But it is downright foolish to lean on an analysis wholly supported by phony photographic evidence. The wise course, as the summit approaches, is to beware of too much zeal -- from whatever source.