Just keep them from getting wet, dirty, kicked, etc. Location is basically just personal/cosmetic preference. It doesnt really matter where you put them, just somewhere that's convienent for you.
Doors are very common, though some doors leak, and get slammed frequently. So i prefer kick panels. I have mine mounted on a very simple display rack in my glove box. So the possibilities are very wide open.
If they are passive crossovers, the only option would usually be tweeter level adjustment. Usually not something you have to adjust more than once. Especially if you can adjust it via H/U.