What I like to do is to make large graphics out of smaller pieces that can be rearranged to make more large graphics. However, this often means that when other people see my tileset, they have no idea what's going on. If we need more mountain or slugville graphics, I can try and demonstrate.
Yes, that's an excellent way of doing things. I do the same thing as often as possible, though not with larger scale backgrounds so much. If you can make some modular scenery of that nature, that'd be great.