I use a sharpie to let me know when I drained the gc. I hold on to it for a month just in case. For cards I use for purchases, I hold on to it for a month and always try to use it for an actual purchase at a place that will drain what's left and it's no big deal. The fast food guys will all do this. Why do I do this? Because places like gas stations put a hold on the card that can last weeks. I've had zero balance cards come back after a month with $75 on it. Same goes for sit down restaurants who generally have tips added. They put a hold on the amount plus as much as 50% and it won't come back for a couple weeks.
I have no issue trying to do a "real" purchase at wal mart with 20 cards in my pocket. Nothing left, no big deal. Something left, you use it for your purchase. Cha-ching.