I don't think this is a large issue. The only time I think it really matters is at the extremes, either someone that mounts multiple amps in a rack, one right on top of the other with no thought as to cooling, or someone that drives the amp(s) at near capacity for almost uniterrupted periods.

My arrangement of channels on a 770 is has more to do with my tendency for geekishness and over-engineering. This tendency has mattered in a quite a few professional situations over the years, where others were not thinking/observing/planning well, but at home it is more a matter of just being consistent in my quirkiness. After all, these are my “toys”, right?!?

I suppose if the 770 were designed such that one transformer handled three of the channels, and the other handled four, then I’ve got more to think about. I am assuming that the transformers share the load more equitably. People operating seven 200's don't have this particular issue, but have others. For me, part of enjoying the results is the process of getting to the results.