I sometimes shoot some older, expired, but well refrigerated C-41 films like Kodak Ultrachrome and the older Portra 160NC. I then usually have them processed by a commercial lab, probably using the current C-41 technology. Should I restabilize these negatives using a formaldehyde based stabilizer, or will they be OK as is?
You will have magenta dye fade and formation of a brown stain unless formalin is used. The films were not designed for a washless process as are modern films, so Ian is correct to point this out.
I too use older 160NC and also Fuji 160S, and always follow the wash with a formalin/photo-flo stabilizer. When I am too lazy to bother mixing photo-flo I use the same stabilizer for B&W film. I cannot comment on PE's remarks because I have always finished color films with stabilizer, both for E-6 and C-41, and (I hate to admit it) for C-22 and E-3.