You shouldn't run into any problems if it's just rims unless it's something very very extreme. IMO if the tires are the same diameter, even a wider width wouldn't be enough to void your warranty. Think about it this way - if you had your rims smashed or your wheels/tires stolen, what obligates you to get the same stock rims? Nothing, that's what. It's your car from that perspective.
The other people here are right, dealers and auto makers are notorious for being bastards about things like this. If you have any chance to buy them through the dealer and have them installed, I doubt there would be any problem whatsoever. Engine mods will almost always void your warranty, especially any kind of nitrous or forced induction. I know a woman who works at VW warranty claims, and VW is very very picky when it comes to mods, even going so far as to inspect vehicles for mods on any type of service and voiding warranties on the spot, even if the requested work was unrelated to the mods! So, be careful out there...and if you want to mod a car's engine, get something used first with solid aftermarket support.