You have put a number of firms through the wringer and picked one. The winner exults, the other firms gnash their teeth. The only fair step is to tell the disappointed firms why you did not pick them and give them enough detail so that they feel they learned something useful for the next opportunity.
This is the test you should set yourself. If you can’t articulate why a particular firm did not make the cut, you haven’t thought through your criteria well enough, reached a shared consensus, or gathered enough information. (I won’t even countenance the possibility that you lack the courage to tell a partner the bad news.) Don’t fob them off with “chemistry.” Tell firms something specific so that at least they understand your reasoning, even if they can’t accept your conclusion (See my post of July 27, 2007: honesty toward disappointed vendors.).