It depends on what it is. Obviously anything that shows any views, or allegiance to any team, would be silly to allow - because it could put the officer at risk. And of course if it can be covered up by uniform then it is unlikely that it would be a problem (and maybe not even known about in the first place). On the whole, though, I strongly believe that if there were equal candidates for a job, and one was covered in tattoos and the other wasn't, it would always be the one who wasn't who would be given the job. I may be wrong on that of course, but I just think that it generally gives a better impression to have a clean body.