Here are the design parameters as I see it.
For the center, you need an amp that is qualitatively at par with the front mains, but you typically don't need the power. This makes a 3 channel for center and surrounds suboptimal becuase typically it will compromise the center channel (or be overkill for the surrounds if you get a very good one)
Nonetheless, If you listen mostly 2 channel with the ocassional movie or MC music disc, going 2 channel (or two monos) for mains and 3 channel for center/surrounds is not a bad compromise (I had this setup for quite a while myself).
However, if you get more seriously into multi channel, you need something better for the center. I would advocate a half decent stereo amp for the surrounds, and three mono's for mains and center (or a stereo for mains and mono for center), with the center having the same sonic signature as the mains but less power. For example if you went class D, you could get Bel Canto's Ref1000M for mains, a Ref 500M for center and a Ref 300S for surrounds.
I personally am using two heavy duty pure class A mono's for mains, a Bel Canto's 300M for center and 300S for surround. (additional advantage is you can discretely put the class A's behind the rack if you are space constained). My thinking is the center does not need the bass slam of the class As, but does need the clarity of the class D, so mixing and matching class D and Class A works very well for me. In fact, if I ever find a single Bel Canto 500M on audiogon, I will upgrade my center amp.
In my view, the key is to get the center channel right recognizing it need to be better than the surrounds, as good as the mains, but without the requiring the same power or bass.
Final thought, a five channel amp is a complete non starter, since your front / center / mains needs are very different (as reflected in your choice of speakers).