I personally like to build interiors, partly because I'm an engineer and I enjoy the challenge of making the interior accessible and "playable", and partly because I like to think that if my little guys were to "come to life" in the middle of the night, they'd be able to get to every part of the castle via either stairs or ladders. I like making my builds functional
That said, deciding whether or not to do a full interior depends on quite a few factors:
1. Do you have enough of a collection to be able to dedicate possibly valuable/limited parts to the interior without scrimping on what the outside looks like?
2. Do you have the time to build a lot of detail into both the inside and outside? I basically figure it doubles or (more often) triples how long something will take to build if I'm going to worry about the inside as well.
3. As others have said, is it going to be for a static display such as a convention or behind glass where nobody will be able to see interior details anyway? I ran into this when building my first BrickCon display last fall.
If you are going to have it around your house to show to friends or are willing to take the time to photograph the inside to share online, it may be worth it to spend some time on the inside though.
4. Will your building techniques allow for a detailed interior? Either because of complicated techniques, or simply difficulties accessing certain parts of the finished product, this can be tricky. Hinges, lift-off sections, and modularity can help, but again, these add time to the build both in conception and implementation.
5. Personal preference! If you don't give a rat's tail about what the inside looks like, don't feel like you have to to please someone else. Most of us are quite impressed by the exteriors alone! Alternately, if cavernous interiors are going to haunt your dreams, well then you maybe should spend some time on it. Even adding simple interiors may give you some satisfaction. Not everyone needs a fully tiled mosaic floor to be happy with the finished product
I hope this helps both you and anyone else who may be facing this dilemma.
Keep up the good work!