To save you some trouble and flames, let me tell you what I found out when I posted the same question last month. Actually, no one flamed me; everyone was very nice. However, I was told that this question is definitely a sore spot and can become quite a powder keg.
There are at least two schools of thought on this:
1. Use them as you see fit. Some people were told when they bought them that the mug was good forever and they stick to that.
2. The mugs are only good for the visit on which they were purchased. You can probably still use them, but you'll be stealing (or, at the very least, be taking advantage of the system).
What it boils down to is you have to make the decision you're comfortable with.
Now having said all that, please don't flame me!