CF4L, the relationship between you and your tattoo artist is complex but good. You have an idea and you work with the artist to help you realize your idea. They are an artist but at the same time they understand that it's your idea and that it's going on your body. It's like an architect or a custom frame builder, they have the talents but they're also working with a client to make sure that what they produce makes the client happy.
So work with your artist however long you need to make sure you're happy. I've always come in with my stuff pretty much fully designed, but the artist has always had small suggestions for changes and things like that.
Any tattoo artist who gets bent out of shape because you're not allowing them to realize their vision on your body doesn't deserve your business.