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Would it be an alright uniform policy to mention to firms that I'm interested in clerking? Or is there a drawback to that?

Firms have very different stances on this matter - some actively encourage clerking and others will ding you right off the bat for mentioning it. I would say not to bring it up until you know the specific firm's policy (you can contact alums, SAs, etc.).

I wouldn't, unless they ask.

yes if interviewing at a lit boutique with lots of former clerks. you might even get some good advice from the interviewer. otherwise, clerking is neutral at best and not worth mentioning. most firms tolerate clerking only because they have to to attract a good portion of the top students.