Tattoos are pretty common here. Basically, keep them off of anywhere below the wrist bone or above the collar (in a v-neck undershirt) and nothing offensive (gangs, alcohol, sex, etc.). Having a tattoo alone won't preclude you from Admission or even be looked at unfavorably, but definitely do your own research and refer to the most up to date policies and regulations. There are not different tattoo requirements placed on us than any other Coast Guardsman.
All of that said, I would still recommend waiting until you're here for the sole reason that if for some strange reason the policy changed between the time you get it and the time you're coming to the Academy and yours was all of a sudden not allowed, you'd be out of luck. If you're in service when the policy changes, you get grandfathered in. The odds that this happens are slim to none, but the risk is pretty big if it does.