I had to write ops manuals for a new Swiss AOC some years ago. The FOCA insisted on limits as per PilotLZ's post, which we put in as:
Cloudbase/vis < 2x minima - AP to be used. i.e for a standard Cat1 approach RVR less than 1100m and cloudbase less than 400 ft
Cloudbase/vis < 3 x minima - FD must be used
Cloudbase/vis> 3 x minima - raw data permitted.
The intention was actually to encourage people to practice by having defined limits rather than a woolly "when appropriate" that leaves people concerned they are opening themselves up to criticism if anytihng doesn't go to plan.