As a subset of total customers? Few. As a subset of total customers that *have problems related to illegitimate code usage*? Probably many. If you are going to assume that people are going to behave, you don't need to worry about anything and don't need specific codes... if you have specific codes, you need to worry.
I am failing to see how you have enough information on *your* end to interpret the information on the customers' end. The customer probably *can't* prove ownership. This brings us to:
If so, why bother? As you don't have enough information to tell which complainer/code user is in the right, I fail to understand the point of the whole shebang.