You can do either... They mostly drink from leafs and vines. So giving the plant and vines a good solid mist so everything is wet will be good. Some chams don't like the water to be on them... some don't mind.
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.