Yeah they do! Always!
Chinatown buses stop at the taxi stand/parking lot outside of Baltimore Travel Plaza going into and out of DC.
Megabus stops at White Marsh Mall, and Boltbus doesn't stop at all, I don't think.
There's another option called MVP Bus that actually stops in downtown Baltimore that has wifi and tables, near North/Charles St., which goes directly to NY for $18, I think. It might be $20 now. Still a hell of a lot cheaper than Amtrak, and way more accommodating than Greyhound.