If you want a tattoo then go for it. It's not like you are having it somewhere visible. I can't even remember if I had to say whether I had a tattoo or not! I would get it very soon tho, as it's pretty painful to bend down and stuff while it's healing...
Within the contract pack, there is a 'new starter' form which has something about tattoos or markings on it. I guess that will just be filled in when we start.