I've heard this many times before and I haven't really seen any studies on this affect, I think there is like one that says it doesn't really matter. I also have a a problem with the concept. It's really early in the morning so please forgive me if I don't purvey my point across in a sane manner. But would it really matter what time of the day you do you're cardio? I mean whether you are doing cardio in the morning to "attack the fat stores" or doing it later in the day and "attacking those glucose carb stores" at the end of the day, didn't you accomplish the same thing? Cardio mid day depletes what easily becomes fat first. ...You know what, it's too early and I'm having trouble knowing what the **** I'm talking about.
Either way I think what is important most is the type of cardio than the time of day. for me it's high intensity for short periods of time. No more than about 15 mins. but sprinting n' **** like that. I've seen more of a difference in my body when I sprint a mile or do "wind sprints" (for those of s that remember that high school ****!!!!) than when I jog 5 miles.