It's my current understanding that if you wait until you are actually thrown, it's too late. You have lost control of the situation.
I've been told that ukemi is about protecting yourself and if you wait until you're thrown then you are no longer in control. You're reacting. If you're ukemi is not as good as you think you can easily get hurt.
Just this past weekend I was told that the real challenge of ukemi is to take your sempai's best technique. This means an honest attack and enough awareness to blend once your center is taken, so that you don't get hurt.