I find generally that tattoos in fiction and specifically in comics are used in a pretty ineffective way.  I cannot say that I am any expert of tattoos, I have never really given any serious thought to getting one, though I guess the thought has crossed my mind once or twice.  In the realm of tattoos that I have seen on comic book characters the majority of them serve one purpose and that is to serve as a conduit for the character to materialize the object or being that the tattoo represents (in some cases tattoos are the souls which have been stolen.)  As I was reading Call to Wonderland last week an interesting concept was used there which I had not seen before.  The entirety of the Wonderland series within the Grimm Fairy Tales universe depends on the Mirror to Wonderland to act as a portal between the realms.  This mirror has proven pretty difficult to dispose of, the main protagonist Calie having broken it on a few different occasions.  Call of Wonderland suggests that other portals can be created with the proper use of the right symbols as long as they are drawn properly.  This is where tattoos come back in.  Whereas tattoos were once considered a mark of certain class of person, they have become mainstream with a tattoos sometimes taking on beautiful works of art, especially as they relate to the human body.  In this case one of the series two protagonists is a tattoo artist and the tattoo she creates unknown to her serves as a portal to Wonderland.  In this interesting development the writers here have created for the first time (at least as I perceive it) the use of tattoos as something more than just the obvious.  It may not be the most imaginative thing ever, but it is nice to see that new ideas are still coming out of the series that I have come to depend the most on for my comic reading.  

Tattoos are sometimes good running jokes like Constantine's tree tattoo.