I've never flown with a cat before, but my $0.02 is that of all the things to worry about, whether of not they "go" should probably be the least of the worries. Suppose she does go? *shrug* so it takes you a little time to clean it up. No biggie, no harm done. Just assume that she will, make plans for if she does, and then be pleasantly surprised if she doesn't. Much bigger worries are things like whether your cat will be safe, how terrified or stressed she will be, and if she won't be in the cabin with you, what will you do if they lose her carrier, like so much of our luggage is.
Here's an article on this from a magazine that I subscribe to:
http://www.catster.com/cats-101/cat-air-travel