I like to buy tickets ahead of time because I can get a slightly better price and because I don't have to waste vacation time standing in line to purchase them.
It did backfire on me once (at Busch Gardens in Wilmington, Virginia -- not Disney). I purchased tickets ahead of time, and they sent me un-activated tickets -- at least I think that was the problem, but it's been about three years ago and I'm not 100% sure. Fortunately, I had saved the receipt that came with the tickets. Unfortuantely, it was back in the car and we had to wait while my husband hiked back up to get it. I was really mad because it took them the better part of an hour to straighten out THEIR mistake, and they were nasty to my husband. We lost the advantage of being there early in the AM, and we could've spent less time just buying our tickets that morning. I wrote and complained later, and their attitude was, "Well, you got in, didn't you?" Wonder why I have not been back there?
We're going to SeaWorld this summer, which is a sister park to BG. I will not be purchasing my tickets ahead of time.