I don't think it really makes that much of a difference when you exercise, so long as you exercise.
I've heard quite a few theories on when the best time is - from morning to night, depending on which article you're reading... So I just figure whatever time is best for your schedule. After all, if you're exercising when you're in the mood, and not when it's prescribed, you're more likely to do it!
But, as it's been said before, just make sure your getting adequate sleep!