Displacement will alter the mesh, and if your displacement value is too high, it will grossly deform the model.  Considering that Mehndi is more a skin deep tattoo, you will  have relatively no height on the surface.  A bump map would bring out some detail and would be the preferred method.  There is a tattoo tute somewhere in the forum which does contain BB's input and comments.  But it is several years old and I don't have a reference link ATM.