We have always sent files that we print regularly then usually have them print solid swatches of gray, red, blue and anything else that may be hard to hit. Send them the most difficult files and make sure they meet your needs. You never want to buy something that can’t meet your quality standards.
I took a whole heap of files to Digitech in San Antonio and they knocked it out of the park. No special treatment, just rip and print. Colors, quality, and adhesion were better than anything else I had seen. I would definitely recommend checking them out if they aren’t on your list. Best kept secret in the industry in my opinion.