If I am hand misting I always make sure I spritz the chameleons themselves a couple of pumps just to trigger the drinking response if they are thirsty- most will smack their lips and start to drink when hit directly.
If I am using my misting system (which is always nowadays) I let the chameleons choose to move in and out as they wish. But it's on for at least 20 minutes at a time- plenty of time to attract a thirsty chameleon to come over for a drink if they want it.