I've done both, pitching a smack pack with, and without a starter. I've found that if the smack pack swells quickly, you don't need a starter. If it takes several hours (like overnight) a starter helps a lot. If it swells quickly, you have healthy yeast. If it takes overnight, the yeast is struggling, and could use the help of a starter.
Again, it doesn't hurt to add more wort to the starter after it's already started. In fact, if your yeast is weak, it might even help get to the proper pitching rate.