I replaced the door/dash speakers and also added speakers behind the seats. I know a lot of people (esp. Audiophiles) like fronts only, but adding the 6.5 behind the seats with the tweeters in the pillar hole really helped round out the sound.
This is my second 370Z. Last time I put the amps behind and underneath the seats. This time, I put them in front of the spare so they are now out of sight.