I'm in the same boat at the moment. I have a commercial client that owes me a few thousand dollars from two previous assignments. On all my paperwork, its states very clearly, "...rights granted upon full payment..."
The reason I did this is two fold. One, I don't want them to have the rights until I am paid, and it also helps speed payment so they actually have the rights before publication.
Secondly, is that it is the most powerful collection tool you can have. Up to $150K in fines per occurrence, file in Federal Court, they could have to pay my attorneys fees as well as their own, etc. Wave that in front of them if they have published the images without paying the photographer, and it gets their attention faster than any collection letter would!
The client I have the issue with now will be getting such a letter in the mail shortly from my attorney. I've been patient with him, its been over 8 months now, and he has broken a couple promises to make good. So its time.
I wouldn't worry about getting a reputation of being a difficult photographer. Rather, you will get a reputation as a astute business owner. I don't see other business being walked on because they want to be liked, so why should a photographer be any different. The client is the difficult one, and any reasonable person with the facts would see it in a similar light. Bend over backwards, and you'll have all sorts of people taking advantage of you.
Go after the money. It was promised to you. Its owed to you. You aren't being the difficult one, he is!