oscroft,
What's the problem with them?
I know a common problem is slow apertures, usually caused by migrating grease at the rear of the lens. That's usually an easy fix by removing the screws in the lens mount (3 or 4) and cleaning what looks like oil from the available surfaces. After cleaning with rubbing alcohol just make sure the thing is clean & dry before reassembling. I don't remember any particular timing for these lenses but digital snaps as you go along or notes/sketches etc. shouldn't be to difficult for most folks.