I want to use 5.1 sound effects in my movie.
Should I use the rear speakers for music also?
Should I put the red dot in the center so the music plays evenly over all the speakers or should I put an effect on the rear speakers (possibly delay)?
You can pretty much do what pleases you artistically.
A good way to learn is to listen to some of your favorite 5.1 channel films and see what they do.
B.
For music, if it's already mixed what you might want to do is put the original mix in the fronts, then also create a track with Left minus right and mix that into the rears. This will phase cancel everthing in the center, leaving you mostly with the stereo reverb information, which is what you want coming from the rears in most cases.