most dealerships would never notice if you got your car tuned unless your engine completely blows up
if it's tuned correctly, that shouldn't happen unless you just flat out abuse your car in which case they would void your warranty anyway. custom dyno tuning is one of the best things you can do for your car, though it is wise to wait till you have all the mods you want to do first. cold air intake, high flow cats and exhaust along with a tune will get you the best results.