Unless of course it has a dong slider your mmo will not make a lasting first impression on the player until you showcase every class possibility. I believe giving the opportunity to the player to experiment with and get a feel for the classes and their capabilities at the very beginning of your game will improve it immensely. I am left pondering why mmo's don't do this more prominently. I've lost count of the times I have read or heard somebody say it about mmorpg's that "it didn't click for me until I played such and such a class."
But what way would you go about it though? Well I think if your mmorpg has horizontal progression there's no need to have a level cap of more than 10, max. Or you could do the most logical thing and just put it around near or in the tutorial somewhere. Even just putting it in before the player hits the character creator would seem most convenient. So long story short: showcasing what your mmorpg has to offer with a deep class tutorial tailors the player experience for lasting effect and enjoyment.