Like Omahahn stated, this topic has been beaten into the ground time and time again by numerous forums from every manufacturer on earth.
With that being stated, I'll leave my $0.02 on this topic.
For me, its always been best to be honest with your choice of dealership and try to build a decent relationship with them. I've gone to the same dealership for both vehicle purchases and service for close to 20 years now and if/when I walk in the door, almost every employee there knows me by name or nickname.
When I had them order my current 16' SHO, they asked me flat out "so, how long until we see it again"... reason being, over the years I've built the type of relationship with them that the service manager will validate my warranty as long as bring my car to them for installation of the parts I choose to put on the vehicle...within reason. This of course meaning, no external fuel source (n2O, Meth, etc...) and basically laughed when I questioned tuning it and tossed the Rocky-Clubber Lang line...."go for it"
Now I'm sure if the car came in on a trailer with number markings on the windshield and a helmet in the backseat, they would start asking questions. However, they really don't care to spend time digging into it, want the vehicle in and out of the shop asap and onto the next one.
If warranty reasons are holding you or anyone else back, stop...and just tune it. As long as you pick a reliable tuning company, you're fine and have nothing to worry about.