As long as its not visible its not a problem, although they can show through some cheap/summer pilot shirts so be aware of that.
I can understand why the airlines are unhappy with visible tattoos because passengers want to see clean cut, tidy, healthy looking men/woman with whom their lives are entrusted too. As an airline pilot, one of the things in your contract is to maintain this image to the general public and it is part of the job.
Most tattoo artist's will not apply ink in visible areas (unless the customers is already heavily tattooed) because it does affect peoples perception.