Depends. If you're doing high volume, then you'll use the paint before it goes off. If low volume, you would need to store it somehow until you need it again.
I can't tell you how many cans of enamel I've tossed because it goes off in the can. Changes in barometric pressure causes them to lift the lid just enough to let air out, then it lets air in and starts the process of "skinning". That's when a hard skin forms on the top of the remaining paint. That's about the end of the paint as far as I can see. It's never the same after that.
So, yes, there can be a huge waste of paint and it can be very costly.
Using the pot and dipper process is a lot more efficient.