If you are just getting letters, the suggestions I like best so far are inner wrist and inner leg above ankle. I like tattoos on the upper arm (I have one and I'm a girl!) but I think that a quarter sized tattoo of letters is too small for that part of the arm. I like something a little bigger there -- and yeah, when I think of the arm I think of the outside part -- watch any NBA game because many players have tattoos there. The inner wrist and the ankle area would be VERY easy for a guy to cover at work or when necessary, although IMHO it should never be necessary to hide your tattoos.
I have never heard anything about which arm is tattooed standing for anything (like sexual preference or anything else) but again, I'm a girl and so I haven't researched it.