Technically putting on infinite fuel/unbreakable joints/infinite RCS is cheating because, well, you are enabling cheats in the cheat menu. Not that I care at all, I use it to mess around with things all the time. (Especially for testing things, I can turn on infinite fuel and fly a tiny rocket to space to test stuff).
But the only one I absolutely would not consider a cheat is collision in part editor. That's 100% awesome, you're going to need it in a lot of cases, and you can make much better looking craft by using it.