The first surround A/V receivers, at least my Sony and others I looked at the time, only had 20w going to the rears. Of coure, this is way before
things got to the point they are now. Later when I upgraded to a Onkyo Integra, it had 105w x 5.
I like having equal power to the surrounds, especially now that we have Dolby, DTS.
I would say another factor to consider, if you listen to DVD-A, you would want to have equal power to your surrounds. We listen to 5 channel
stereo a lot, it works for us.