I wouldn't want super tight heel cups to the extent that I feel they exert pressure, myself, but the shoes I've tried, the heel cups didn't seem to bother me (I don't have big heels, I guess). My "limiting factor" in shoe fit is that I need a good bit of space in the front for my toes (I've lost some nails after my last IM which I attribute to a too cramped cycling shoe in that area), and shoes which accomodate that tend to be comfortable enough with the heel, too.
I've got a pair of wide fit Lake CX301s lately and while with a single Boa dial and a very lightweight shoe construction the retention isn't as good as a firmer shoe with two Boas, they're the most comfortable thing around and I haven't noticed any disadvantage in power resulting from that, not even in a sprint (that said, my "sprint" is a sordid 1000W affair, so... yeah, hah hah). The big thing is to be comfortable, imo, especially for long distance.
I would imagine that heel being gripped firmly is very necessary if you're really dropping your toes in a very pronounced way (which could be compensating for cleats far forward plus saddle up, or natural tendency).