Trading your stronger Pokemon over to a new game file to make beating it easier isn't cheating, as long as you didn't cheat to get that Pokemon in the first place, heh. But wouldn't that be kind of pointless, seeing as you'd need all of the gym badges to make a level 100 Pokemon obey you anyways? Or did they remove that?
I'll probably catch all of the Pokemon I can in Platinum, and then breed whatever I'm missing in Diamond and transfer the eggs over, although I might just set up my team in Diamond and then send the eggs over, hatch 'em, and play the entire game with them. Why eggs? My PC in Diamond is almost perfectly set up with every single Pokemon, 1-493, only, like, 21 missing, I don't want to mess it up :P