What car speakers should I get?

My car has 4 speakers. If I get 2 coaxial speakers do I need to worry about getting back speakers? Do most people get 2,3, or 4 speakers? If I get just the two coaxial should I get an amp that has only the wattage to power those two and just let the stereo directly power the back two? Then should I be wary of how much wattage my new stereo head puts through so that it doesn't blow out the back speaks? Let me know what you think! Thanks!