When you come to a stop light or anything else, if you won't be stopped long, doing a trackstand is convenient, and it's also a decent way to improve your bike handling skills. But, if you're going to be stopped longer than you can balance the bike and not move it - say a train is going by - always unclip the same leg to stand on. When you need to do this in a hurry, you want it to be reflex, and not something you have to think about. You want to be able to get out of either pedal quickly, but you want to get in the habit of doing it with the same leg for routine stops.