It really depends on the ability. Average damage is mathematically "half" which I think makes it underwhelming. I regularly roll far above half when I use a dragon's breath weapon. EX: The dragon's breath weapon is 12d10, 120 maximum. The "average" is 66 (5.5 per die), that's also what you would save at if you took maximum damage. When I roll, I generally roll between 80 and 90 damage.
I don't mind using a fixed number for lower dice rolls, it saves a lot of trouble when running numerous monsters. But I feel like it really cheats the larger creatures of the OMG HOLY WTF!!! factor, so for them I usually use average x1.5 It brings breath weapons up into the "this is a real threat to your life, even if you save it still hurts!" Which is where it should be.