Like Reece said, the earlier the better. Max results will be early mornings. This is because your cardio rate will increase during any workout routine and will slowly decrease over time following the routine. It should give you more energy throughout the day but also call for a short nap later on in the afternoon.
I like to do mine before I go to bed. I feel like I'm burning off the calories from the day and that it tires me out for a good night's rest. After I work out I take a shower and hit the sack. Is there that much of a difference between working out at the beginning of the day and the end?