I've only used liquid yeast once, though I plan to use it more in the future.
Since I only bought it a day before brewing, I simply pitched the White Labs London Ale vial into the fermenter. The fermentation started, but it took about 3 days. I had already rehydrated some dry yeast to pitch when I noticed the airlock was bubbling.
From that experience alone, I'll create a starter for my vials from now on. Even if they say they are pitchable, I would recommend a starter because they take a while to start. I've read that it is best to get fermentation to start as soon as possible to decrease the chance of the wort getting infected.