Grounded means a three prong plug...the one with the round third prong in addition to the two flat regular ones. It protects the device when you're using high amounts of energy. You don't really need it since you only have one cage.

And, I would still place the Acurite probe under the warm hide just to get a good comparison temp constantly. As for positioning for humidity, you'd still want to place the unit at the back center to get a general reading.