Also, kind of related but not really:
I don't think that completing parts of the main quest keeps you from doing sidequests, but completing certain sidequests will make other sidequests unavailable. Turning Doctor Badger or Penny into a bimbo, for example, can limit your future interactions with them and make it impossible to complete certain quests.
So just be wary of that if you want to do as many sidequests as possible. I keep lots of saves and check the wiki a lot.