NOW THAT THE FEDERAL bribery-conspiracy charges against former Rep. Edward A. Garmatz (D-Md.) have been dropped , an obvious question is whether Mr. Garmatz should have been indicted last August at all. One can easily argue that if the federal prosecutors in New Jersey had exercised proper care, they could have found out earlier that their key witness, United States Lines President Edward J. Heine, had apparently lied and fabricated crucial documents. If this had been learned last summer, instead of being ferreted out by defense attorneys and prosecutors last month, Mr. Garmatz could have been spared the strain and public discredit of being unfairly accused. And the government, not incidentally, could have been spared considerable embarrassment.

How badly did the government goof? Mr. Heine had claimed that Mr. Garmatz, while chairman of the House Maritime Committee, had taken bribes from shipping companies. Surely the government should not have brushed that off just because it came from a man who was under investigation for involvementin an overseas slush fund, and was trying to save his own skin. Most evidence of official corruption necessarily comes from people involved. Granting immunity in exchange for testimony is a common and in indispensable means of pursuing such intricate crimes. The Garmatz fiasco does not invalidate that technique. But it does underscore the risks and the need to examine evidence from such sources thoroughly and skeptically before proceeding with a case.

Without studying all the facts and alleged fabrications, it's impossible to say just how badly the prosecutors were initially misled. What matters more, in any case, is that they did reassess the situation and agree with Mr. Garmatz's attorney. Arnold Weiner, that a case so riddled with problems should not be brought to trial.

Mr. Heine, whose own legal troubles have greatly increased, still maintains that he has told the truth. The other day his attorney, objecting to the dismissal, said the courts and juries - not United States Attorneys - should resolve issues of credibility. That's half wrong. Of course judges and juries render the final verdicts. But if prosecutors suspend their own judgement, they can become the pawns, and innocent people the prey, of anybody who, for whatever reasons, wants to sling accusations around. Russell T. Baker Jr., the deputy assistant attorney general (and prospective U.S. Attorney for Maryland) who made the decision to drop the case, summed up the proper standard when he said that "the government will not sponsor a witness whose testimony is untruthful in any respect" and will not proceed with a case that lacks "integrity." That is what justice requires - and, however belatedly, justice in this case has been well served.