I dont think there is a "perfect" time to get cardio done. Personally, I wouldnt want to do cardio immediately before training..Id rather use that energy toward lifting. In a perfect world, I would much rather do cardio opposite of my weight training (train in morning / cardio in the afternoon or evening), but the bottom line is if you need it, just get it done. Life gets in the way and you just have to do adjust. So, if I have a flight and want to get in the cardio...I will get to the gym a little earlier, lift, and then knock out some cardio...and head to the airport.
For me..on training days, cardio opposite of training (as stated above) is what I prefer and on non-training days, first thing on an empty stomach.
Good luck...
-s