During a training ride it's no problem to take an extra or two, but during a race do most of you go for a new cartridge or use an old one you have around?

So is there an effective shelf life to these? If so; I suppose I should mark them with the date of purchase. I can't see why there would be a shelf life, they are not suppose to leak. Maybe during enough heat/cool cycles in a car, etc, they might loose some pressure?

Tom