After just getting a tuner, my advice is to be very careful as soon as you load up your tune. In fact, I would even datalog and go 75% throttle in order to be sure everything is kosher.
I know a lot of people swear by them, but every car is different, and if you live in a hot, humid area like Texas
-- I live in Georgia -- you could expose your engine to some harm if you're not careful. For example, I have had pinging with both my first tune as well as the "revised" tune. You may never experience a bad thing at all, but just be very sensitive to what your car is telling you.
That being said, a tune for an automatic transmission -- I have a 2011 V6 AT -- is pretty awesome. You will pull harder; you'll stay in gear longer. It really does make a different car. As far as the warranty is concerned, that gave me some pause too, but if you're careful with making sure your tune is within the proper parameters, you should be fine. Just make sure you keep the rev limiter stock and listen for any potential detonation or pre-ignition. That's the kind of stuff that could damage your engine and be traced back to a tune.