we do 90% custom work / 10% contract. less than 5% reorders of same design would be my guess. we blew through about 850 jobs last year and i'm trying to keep paperwork to a minimum.
for complex jobs - we do keep samples. Ex. 4 color process its important to match it and know mesh, # strokes, etc. to reproduce exactly.
for spot color work, most of our customers can't tell the difference.