While convenience ultimately prevails and you may be better off working out whenever your schedule allows, here are some things to consider.
The average person is about 25% hypoglycemic upon awakening. This could be good or bad. Good because the resultant insulin levels are low from a hypoglycemic state and you are more lipolytic naturally (favorable to burning fat) than any other time of day. Bad, because your core body temperature, your energy level (and quite possibly motivation) is low and your joints are stiff (especially if you are over 40).