Nope, it's not cheating. As someone else said, it's just changing a game setting. The game is meant to be played how you want to.
I've used motherlode to edit different households and to add concepts that hadn't been in the game. I'll use less of it now that I have an established save and we have a better way to give inheritances. I never felt sorry for that. Not all of my households were "rags to riches".
I've never really cheated needs. I have occasionally added a specific reward trait for "story-telling purposes" but not often. I've heavily edited sims in CAS whenever I want to during gameplay.
The features including the cheats are made to be used or not. Lengthening the lifespan just means that you want to spend more time with a sim. With my large rotation, it is as though sims are already around for a long while so I don't play on long. I'm also not specifically attached to one household. I play on normal but I pause aging a lot so mine is a customized approach.