In my last pickup, I ran four MB Quart Discus speakers along with a Rockford Fosgate sub and amp. IMO, best setup to possibly have.

In my 4runner now, I actually have the factory speakers going (all 4 are amped), and they sound great. Not as good, but... I still run the same Rockford setup for lows.

Now, I had strong speakers only in the front one time, but personally I like the sound filling the whole truck. I don't want music blaring in my face all the time. Surround sound is nice to listen to, and decreases the power/wear for the front 2 speakers.

Ideally, You do want to amp all your speakers. It prevents the speakers from playing any lows which provide a crisper sound, and allows you to run a sub. It also protects the speakers from damage.