Someone has showed me a few pieces of mine that have been graded by grading companies. Since it's my work I'll just say this is unneeded, grading original art is pointless as it's already completely unique. If you want me to verify if something I made is legit, then contact me and I'll tell you (for free!)
Otherwise don't bother wasting your time and money, you gain nothing by grading something that there is only 1 of in the world already.