Why would you want to put speakers in the back anyway?
Your whole sound stage will be screwed, especially if you put bigger(louder) speakers in the back.
I have a set of splits and a sub. Bass is non directional and will only fit in the boot. You want all of your music to come from the front, not from behind you. I reckon this is all you need. By a few GOOD things rather than lots of shit.
Do you go to a concert and face the back so all the sound comes from behind? I think not.
Those who posted their theories on wood in spacers are right. It will resonate less, therefore absorbing less energy from the drivers and sounding fatter and deeper. This will go for a parcel shelf too if you decide that you need rears. A MDF parcel shelf will sound heaps better than the thin bit already there.
Oh, and if you think 6x9's are the shit you should trade in your Skyline and get a WRexcel.
And if you think I don't know what I'm talking about, have a look at my posts here for pic of my previous car.