I've never purposefully time travelled, I could never lie to Isabelle, the pure cinnamon roll. But accidentally when I forget to change the time for Daylight Saving.
I usually only time travel if I really feel like I need to do something, but missed the time slot. I usually only go back to noon of the same day, that way not much has changed. I used to have a cycling town though and time traveled quite often back then.
I haven't, because I'm too afraid of messing things up in my town. :S When I've got all the furniture I want and played through all the seasons and events, I might start.
I usually time travel to try to move people out, get PWPs, finish PWPs, house reset, TT earlier for stores to be open, TT later for the island, and well, I just TT a lot!
For while when I barely played ACNL, I would time travel big amounts backwards so that I wouldn't lose any villagers whenever I'd load up the game. Now that I'm trying to get back into it, I don't time travel as much.