They dealer's only reason is to make money; it's how they stay in business. The more work they do for you, they more money they make. And that's fair.
When it's not fair, is when they start pushing things that you don't need. That's called cheating, and should be a red flag as to how the business is run.
Anyway, to answer your question: avoid warranty hassles by keeping a written record of the work done. Make sure the work done is, at least, what is specified in the "passport to service" booklet. That will more than cover you. Note this applies no matter who does the work. You can not be denied warrant coverage because you did not do the service at a dealer, provided it was properly done.
Even if you had done *no* service, the dealer would have to show the lack of service caused, or is likely to have caused, the problem. Thus, if your radio malfunctions, it doesn't matter if you changed the oil or not, but if your engine freezes up and you can't prove you've changed the oil in the last 48000 miles, well....
Thanks,
Shawn