Rear fill wattage ??'s

I am looking to use the Focal Utopia 165W's for the front and the Focal Polykevlar 165K2's for the rear and (1) 12" Cerwin Vega A.I. series subwoofer. I found that JL Audio has a 500/5 amp which puts out 100watts to each front channel, 25 watts to each rear channel and 250 watts to the subwoofer. Is there any guidelines as to how much wattage the rear speakers should have as compared to the front speakers? I do have a couple amps left over from a previous system that I would have to check the power ratings on. One of them is a Pioneer Premier GMX-904 and the other is a Kenwood 2 channel.

In a well set up minimalist system there are no rear speakers at all so there's no need for any amp power. In a system with a well set up 4 channel setup you would only have midwoofers in back that have been bandpassed. Since the rears are for "rear fill" there's no need to have as much power going to them as the front speakers. But if you just want to be as loud as you can possibly be then send as much power to as many speakers as you can afford.