I am just getting into liquid yeast, so I'm no expert. However, I'd say it depends on how old the washed yeast is. The yeast viability decreases over time, whether its in a vial, smack pack, or a jar of washed yeast. When did you wash it?
(~):} Just a little Furthur (~):}
Go here and select re-pitching from slurry. If you have enough "slurry" you won't need to build up your yeast with a starter, but as mentioned by wendelgee, you may want to give them a bit of wort so they wake up. Depending on how long ago you collected the yeast, you will also be verifying that they are indeed still viable.