This all has a hell of a lot to do with how you deal with people, and what sort of people your dealership employs. I have never had any problems with warrenties, despite modifications. If you walk in to your dealership the first time to buy your car, and you bother the hell out of the salesperson, then, on the first schedualed maintanance visit, you are content when no one recognises you or asks you how things are going lately, you will have problems. I went to a dealer on the opposite coast once, two years ago, and they still remember me. People want to help those they consider friends.
On the other hand, there have been places containing people I couldn't help but insult. I once demanded from a manager that his incompetent mechanic give my keys back before I placed them in an area where he might find an intimate relationship with them. It's a two way street, but most of the time, if you can see past the nametag, theres an interesting person back there.
Cheers,
Kennith