Shoot, Vern, it used to leak oil...
Regardless of what you think you know or have read, the OEMs certainly can stipulate a very rigid series of maintenance operations and even usage -- for example, break-in and no racing are two that you're commonly aware of -- that voids the warranty. As it should be: abuse or misuse something and you shouldn't be able to bring it back. On top of that, warranties are also governed by State laws -- for instance, California has a lengthy "Lemon Law" process that requires dealer service/investigation. It doesn't work very well. We've had lengthy email discussions with many readers' experiences with warranty problems (in many instances, we've tried to mediate with the person and the OEM behind-the-scenes) -- one even culminating in the person driving to the OEM's headquarters and leaving the bike in front of the main doors, tossing the keys inside... literally "their" problem from then on!
While doing all your own maintenance, if you make one single error, poof! Warranty voided. And yes, if you can't prove regular scheduled maintenance, you lose your warranty. Do your own maintenance, drop a valve, it's a subjective call from the OEM's dealer and then possibly attourney whether your warranty is honored or not: If they believe you're a dimwit and screwed it up working on it yourself and you can't prove you have the (certification helps here) skills to do such work, you're out of luck. And if you are a newbie and messed up, rightfully so. It's not on their nickel you should be learning to wrench.
A simple example: Check your plugs and forget that anti-seize each and every time and strip a plug hole? No warranty, you shoulda read the fine print. Check your plugs a lot and use anti-seize and your head had a cooling problem that made the aluminum overheat and become brittle and you strip a plug because of that? (And here's the ringer...) The burden of proof is on you to prove you're qualified, did everything right, and there was an assembly problem or flaw from the factory that caused the breakage. Have the work done by the dealer, and it's their problem.
Thus, we maintain that, for a new bike that is under warranty, unless you're a truly good mechanic who is well organized, it's a good idea to take it to a qualified dealer and spend the service money and be carefull to keep all the service records -- dealers won't do it for you. Just in case.
Used bikes, well, they tend to run their very best right up until they blow up. Valves sure do work well with almost no clearance, but when it gets to zero and rides the cam journal all the way around, you're toast. Not checking valves is a really bad idea (someone above mentioned not needed to check valves on a Magna)!