It all about what you want (now and in the future).
If I wanted to keep my full GM warranty, I wouldn't get my car tuned. I'd probably keep it bone stock (with the exception of a catback exhaust for sound).
But I love modding my cars and I love drag racing. I also knew I was getting headers, intake, etc in the future. So I got my car tuned right after getting it. But my car is an L99 A6 and they show a pretty nice increase in performance with just a tune.
The LS3 M6s don't benefit from a tune as much as the L99 A6s.
My L99 A6 went from 313 ft lbs to 338 ft lbs with just a tune.That's 25 ft lbs. I also picked up around 30 ft lbs from 4500 - 5500 rpms. Trust me, that's something you can definately feel. That along with firmer/faster shifts makes the car much more fun to drive (and just flat out faster).
It's also important who does the tuning. Not every tune is equal. I got my car tuned my Jeremy Formato (
www.fasterproms.net). He really knows what he's doing.
So it comes down to what you want out of your car, and if you're willing to face the consequences if something happens. For some of us its more than worth it.
But for some others, it might not be the right decision.
If I had an LS3 M6 and I wasn't planning on getting any serious mods in the near future, I'd definately hold off on getting a tune.