Most People will say first thing in the morning on an empty stomach, reason being is because your body is short on glycogen supply and will take only fat for fuel for the workout. The point of doing cardio is to alter your internal hormonal environment increasing your metabolism, not burning fat during the workout. There are several important reasons why you should not do cardio first thing in the morning. One, you are more prone to injury, you just woke up from a 6+ hours of sleeping, your body needs warming up before doing intense exercise. Two, your intensity will be decreased, which is extremely important for a good hormonal excreting cardio workout. Three, when you wake up in the morning the hormone cortisol is raised which is the "main" catobolic hormone, which goes away after eating breakfast. Breakfast jumpstarts your metabolism for the day, skipping breakfast and doing cardio instead raises catobolic hormones even more and slows your metabolism, which is a good formula for losing muscle and gaining fat.
Once you get below 10% bodyfat, morning cardio *might* make a difference. Mainly because it becomes so much more difficult to mobilize/oxidize fatty acids the leaner you get.
If using fat for fuel (the premise upon which morning cardio is based) was really that important for fat loss, than interval training (which relies solely on glucose/glycogen for energy) wouldn't cause fat loss which it obviously does.
For the average person, in a more average bodyfat range, I doubt it makes any difference at all: the best time to do cardio is when you will do it consistently.