Just like Prodigy said, it doesn't matter. You can choose any class you want, so you could choose something like Guardian first and then Sith Lord, the opposites. The natural progression is:
Jedi Guardian = Jedi Weaponsmaster/Sith Marauder
Jedi Sentinel = Jedi Watchmen/Sith Assassin
Jedi Consular = Jedi Master/Sith Lord
But that is just the natural progression; You can choose anything you want, regardless of what class you are now.
EDIT: Arrrgh! Darth333 beat me.