Don't think it makes a difference. Garmin gets the absolute elevation from the satellites, then uses barometric pressure to get the difference in gain. I've sometimes nixed the mount altogether and just tossed the Edge into my jersey pocket and still get readings OK.

That being said, I do find that Garmin elevation gets off if you don't let it "warm up" enough before hitting start (that is, you're standing still and the elevation is still changing even after locking into the satellites), or if a weather front moves in.

-- gnat! (movin' on up)