I think that now, particularly with University out, that it's kind of necessary to play with longer life spans in order to fit college into a Sim's life. I personally play somewhat rotationally with aging off because I just don't want to be bothered racing the clock to do what I want to do with all of my Sims. And I play University by taking only two courses per semester so my Sims can squeeze in a little bit of party time and some irresponsibility now and then. If I had aging on, my Sims would never get out of college, lol.
So no, it's not cheating at all, especially if you want educated Sims.