I don't trust anyone to draw anything on me. There are very few instances where the person has good intentions while drawing on you. Usually they're looking for an excuse to draw a penis. You're generally safe when you're at a fair and there's one of those people who does face painting, but even then I've walked away with a penis on my face after I asked for a worm.
When someone is doing a part you can't see, like your back, you have to watch out. They're going to get you. If you hear giggling at any point, abort. It's a trap. Consider never getting a back tattoo without a mirror setup so you can see everything. The last thing you want is to sit on a table for four hours and then reveal that you have a permanent penis on your back.