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It would depend on what services you are providing. Graphic Artist is a pretty broad term. If you are looking for illustration work, perhaps you would include sketches. You could use them to demonstrate how you think/work through a project. Other than that, I think it makes you look less professional - like you have no real work for real clients to show. Your best bet is to look up other freelancers who are doing the same type of work you are and see what they have posted in their portfolios.