hey Robert,
I think you know the answer.... its a trade-off of heat, power cost, whether you feel green, and sound.
Lets begin with the fact that being class A vs AB is not the most important determinant of sound quality. A high-bias AB design does 99% of what a class A design does, at a 75% power/heat/cost savings. And a well designed AB will sound better. Only when all other factors are equal does the last 1% matter. keep in mind also that "AB" covers a wide range of bias levels. Either you understand or i cant explain it succinctly here (but am happy to offline).
if you are talking tubes, its a bit clearer, but would one leave tubes on 24x7 anyway? I hope not.
Maybe I'm falling bait to a joke, but listen and evaluate against the practical impact of electricity and heat. I personally cant justify wasting that much power since my amps achieve the vast majority of their warm up in about 30 minutes. But then again, it was a design goal.
G