Should I ask the graphic artist (for label) to sign a non disclosure agreement prior to hiring?
Yes, and also obtain a work agreement.
I would say it matters what your are hiring a graphic artist for. If the end product is not something they could do themselves than there is really no need. For instance, if you are looking for a hot sauce label to be made: than there is really no need because most likely they could not make the hot sauce themselves even if they have the greatest label in the world. If you are making a website that could generate income than make them sign it, because they might have the ability to steal your idea and create the website themselves (look at facebook)...