I like to keep 3 or 4 of them in the grow room so you can get a global idea of what is going on. I have found that having colder temperatures at the floor and warmer temperatures above the plants is bad and can cost you ounces of sweet buds.
I have one above plant level, one at the floor, one where my air comes in my room. I like to keep them all within 3 degrees of each other.