Rarely do I CDG during a battle, either as player or as DM.
Some illustrating exceptions:
-During one battle, a shadowy rat-form rose from a corpse and demanded of a PC, "Are you the one? Are you he?" The PC, having no idea what the rat-form was talking about, shrugged and said, "Uh, sure." BZZT! Wrong answer. It focused all its attacks on him, knocking him to 0 strength, and then tried to CDG him. Thank God for good fort saves! [although it was similar to a shadow, it wasn't quite a shadow -- thus, dropping the PC's str to 0 didn't kill him.] MORAL: A villain will try to CDG a PC in the middle of battle if pursuing a deadly grudge is more important even than personal survival.
-During another battle, our party faced a nasty fighter-guy surrounded by about two dozen flunkies. We focused all our attacks on the fighter leader, taking him down, but one of his allies dragged him out of the thick of battle. Fearing that he'd be healed up to consciousness, I tumbled through our enemies and stabbed the leader one last time, killing him (not technically a CDG, but effectively the same). MORAL: When a group has a very strong leader, killing the leader can demoralize the followers enough that they'll scatter.
But otherwise, we tend to focus on standing, fighting opponents before focusing on the helpless ones.
Daniel