I have an 04.5 dmax with a predator and my dad has a 02 dmax with the edge system. I was told when you take the truck back for warranty work you should unhook the batteries for 20 minutes and this will "reset the computer" and the dealer wont know you were using the tuner. Is this true?

Just reload the stock program before you go in for work. It is just as simple as loading Diablo's perf tunes.