If I bid on firms that indicate no specific offices--firmwide listing--will the interviewers ask me which office I prefer in the interview?
Yes, they will.
Should I indicate that somewhere? Or should I accept any office?
An explicit indication is not really expected at this stage, but I suppose you could add an "objective" line to your resume if you wanted.
At the interview, I think it's best to specify one or two offices you'd prefer. They need to pick an office to send you to for a callback. Saying "oh, any office is fine" suggests that you don't know about or care about the firm and doesn't make their decision any easier. When I've waffled between two offices in the past one firm really pressed hard for me to specify a single office.
You should choose carefully - some offices are small and/or don't have much patent practice, and if you say you prefer those offices a callback is less likely.
Something like this is what you should aim for: the firm has a large/growing in Megalopolis in your technical area, so you say "I'm particularly interested in your Megalopolis office because your Megalopolis partner John Smith's sprocket litigation work and your Megalopolis clients like Spacely Sprockets would fit well with my background in widget design. I really loved my time in Megalopolis working for Cogswell Cogs. If your firm has a special need elsewhere, though, I'd be happy to consider your other offices, perhaps Smallville."
Some others like finnegan or fitzpatrick do not say any thing specifically.
Finnegan's largest presence is in DC, so in the interview you may want to focus on that office. Fitzpatrick is mostly NY, with a small DC office and a tiny CA office. If you want Fitzpatrick your chances may be best if you say you want NY.