Good point. That's exactly what crossed my mind also. If there are going to be any differences in between channels of a multi-channel amp, you would think the manufacturer would take this into consideration when labeling and instructing the user on how to properly set it up.

I use multi-channel pre-amps at the studio I work at and I can tell you there are differences from channel to channel in relationship to how close each channel is to the power supply. The closer you get to the power supply, the more noise is inherent in the signal at high gain levels (ribbon mics). I would think the same holds true for multi-channel power amps. The manufacturer would probably arrange the channels in some sort of relationship to the power supply to maximize signal quality. Of course, I could be completely wrong.

As for monoblocks, unless you listen to tons of multi-channel audio, I would guess there could be a difference in heat between channels. However, I don't think that difference would be enough to change stacking arrangements. I would just stack according to manufacturer recommendations. I think in Outlaw's case, that's well stated in the manual.

As for rotating amps, that makes very little sense to me. You'll get used to the sound of each channel pretty quickly. If you change out the amp, you could be changing this sound, even if it's the same model amp. Even if it's a matched pair you're switching, there would still be the possibitly and the very likelyness of audible differences.

I would keep the channels the same. At worst, if the front channels happen to fail, you know you have a less used amp to switch in its place.