In nearly every (a)RPG I try to actually play a character to the top, or at least close to the top, before I start another one. I want to keep the game fresh and still have something new to do for as long as possible. That said, nearly every time I fail miserably and I end up playing everything that's available within two weeks of buying the game. In WoW I have about 40 different characters. I really need some self-restraint in this.
So to answer your question, I will most likely try to play one to the top (I'm leaning more and more towards starting with the Monk) but in the end, I won't and will level all classes simultaneously.