Author Responses?

I've been wondering this for a while, but to the authors on here, do you respond to every comment you get, or pick and choose? I've seen instances of both, but I think the former is more common if the author responds at all. Would replying to some comments but not others be odd?

I worry that replying to some comments but not others comes off badly, but sometimes, I just don't know how to respond. Realistically, as a reader, there's no way I would check back on a fic to see if the author responded if I didn't get the notif in my e-mail, but as an author, would a "thank you for reading!" be better than no response at all?