It is safe to get a tattoo behind your ear. Choose a licensed, experienced tattoo artist. It won't hurt too much. In fact, there are not that many nerve endings back there. On the other hand, any neck tattoo is difficult to hide.
I read this before I got mine last night, so I thought I'd share my experience. I got mine along the curve of the back of my ear. I can't say at any point that it hurt, it was just slightly uncomfortable.
I can tell you from experience that it didn't hurt very much at all. There are few nerves there so it didnt cause a lot of pain. The noise is annoying at first, but not really bothersome. So no. It definitely did not hurt a lot.