My wife is an artist, as well as a tattoo artist. Most of her clients come with some rudimentary drawings, or pictures of existing work that they like the feel of. She then adapts them to suit the client. It doesn't hurt to have the basic concepts sketched out, but you should leave the actual designing to the artist, as they will work within their own strengths to make the design the best it can be.
Best of luck on the ink!