I don't believe grout is waterproof. It doesn't stop the water from passing through it. It absorbs water and allows water to pass through it, just like cement or other porous material. Although it may take some time for water to seep through it, it isn't waterproof.
As long as you mind if the grout your using on your backing is waterproof, I don't think it will matters if you have the grout seal or not, in a tarantula cage.