If I have the time, and the starter is "done" with enough time beforehand to decant, I might (especially if it's a light, delicate beer), but most of the time, I don't. Last night I pitched about a 3 quart Thames Valley starter into barleywine wort...just because I stepped up the starter on Friday and it wasn't done in time. As to what Menschmaschine said, which is a good point, what you should be doing is anticipating, while you're crafting the recipe, whether or not you'll be decanting. That way, if you're not decanting, you can include the amount of extract from the starter in your recipe calculations.
The other consideration is the acclimation process. Decanting involves chilling the starter down in the fridge, then warming it back up, then pitching it. It seems to me that you're bound to lose a significant amount of cells this way...with an active starter, they're already actively fermenting (hopefully close to the same temp as your wort) so there's not as much of an acclimation issue.