1. If I cheat during progress it's of sole curiosity, done for a short amount of time, and I never save.
2. I cheat after having finished a game if it's fun, like moonjumping in SM64.
3. I avoid using guides as much as possible, unless I'm VERY pissed at not getting past something(trying for 2-3 days like mad), finished the game and wanna get all the secrets, or I sense a split event approaching that could later prevent me from getting important content, such as rare items or good ending scenarios.
And this isn't just with DS games, it's with all games.