If you have an after market head unit they push out a lot more power than the stock head unit. If you use three way speakers, which are nice, or a two way, like a rockford, you have to make sure that the magnet isnt to big so that your window will still roll down. If you want to mix it up and put 4 ways in the rear that is nice, but they have NO BASS, so i only recomend this to my costomers if they are going to be going with subs also.
I personally dont use amps on my inside speakers, only on my subs. I always spend a little more on the speaker, and save on the amp and wire kit and it turns out good for me. I had a competition 9kind off0 system with 2 15 inch subs and no amp on my internal speakers and it sounded so crisp and clear, I loved it. Juat another point of view for you.