It sounds like what you're asking is would it be better to run 4 speakers at 35W each, or to bridge the amp to two-channel mode and run the fronts at ~70W each and run the rears off the built-in amp of the deck?
Personally, I'd go with option #2. The rear speakers are for "fill" and add a minor effect to the soundstage....you won't need much power the get the ambience they provide. The extra power up front will be a welcome thing....especially if you are trying to integrate them with a subwoofer. The sub might overwhelm the fronts if they are underpowered (35W or so)....
The only other suggestion is to explore getting some "bass blocker" type filters for the rear speakers. Since they are for "fill" they shouldn't be trying to play bass anyway....and bass takes a lot of power that you won't have off the deck anyway. Running only mids/highs through them will make things a LOT easier on your deck and they will sound cleaner too....
