I'm doing the same thing i.e. 2 channel amp for the mains and 5 channel amp for the center and surrounds. I'm using my two spare channels for subs, which works out great for me. If you don't have subs or the subs that you have are active ( built-in amps ), i would recommend bi-amping your mains with the two spare channels. So long as the amps are from the same manufacturer and share similar gain ratings, you should have no problems with this. I would be doing that myself, but my 2 channel amp driving the mains is already capable of 1200+ wpc, so i really don't need any more power there.
As to Lps2cd's comments about being able to combine channels, i agree that it is a nice feature. Depending on how this is being done though, there could be some differences in sonics involved. One would have to try it both ways to see if the added power was actually of benefit compared to the differences in sonics, if there were any. Sean