I've tended in 5e to use average, although I might vary it with slightly different numbers if there's a large group, just for variety.
I have been thinking, however, with playing around with a system with a sliding amount of HP based on CR, just to make monsters tougher as levels increase so as to make up for any magic items the characters might have. I've been thinking about a formula of Average HP + (2% x CR) - so, for example, a CR 10 creature would have Average HP + 20% Average HP. This won't hurt low level parties against very low CR creatures; in most if not all cases, even CR 1 creatures won't gain a single extra HP. Very high CR creatures might very well come out above what would normally be their maximum HP, but since that's like CR 25 and above, that's just part of the challenge in taking such powerful creatures down!