Play where you, and your bankroll, are most comfortable. I hop around in limits/ tourney buyins all the time. I have "niches" where I concentrate a good % of my play, different ones for different sites, depending on my BR situation per site, but I'll move up, or down, depending on how I'm feeling on a given day, how long I intend to play, oh, lots of factors.
I only rarely go out of my "comfort zone" though, and then only for a specific purpose, like a really juicy tourney coming up.