The government, here in the US, mandates that consumer amps be rated with all (I think it's ) "related" channels driven. At any rate every amp manufacturer interprets the regulation to mean that they can rate with left and right or any other two channels alone, and they do so.
, no receiver has an adequate power supply to support full power output to all channels simultaneously. That would be a real problem if (a) in YOUR room with YOUR speakers at YOUR listening levels you ever need the full rated power of the amps at all and (b) there was any movie or music content that came close to having full output on all channels at the same time. You might qualify for (a) but (b) doesn't happen, AFAIK
. Remember if the surround channesl are "one notch" (3 dB) quieter than the mains, they require no more than one half the power the mains are calling for. While the tyranny of doubling power for 3 dB (or 10 times power for 10 dB - twice as loud) is painful and potentially expensive on the loud end, it's kinda forgiving when your SPL needs are lower, as changes that are surely perceptible but hardly noticeable in the heat of battle cut power demands significantly.
In the end, especially since everybody plays the game the same way, either you have enough power to achieve your listening levels without audible distortion or you don't. If you do, whether you'd like to see the power specified differently is pretty much moot.
AAFAIK, the only way to know for reasonably sure how a particular receiver reacts to high power requirements for 5 or 7 simulteneous channels is to find third party testing. It's what I did, once I started hanging around here and got the mesage . . .