The recent best advice is as Womble says - don't trim routinely, treat any foot disease then tidy up afterwards if necessary.
This research however, was carried out on large flocks, where the sheep are brought in at regular intervals for other reasons, but have their feet checked at the same time, and trimmed 'to be on the safe side' until the next gather. This caused a spread of disease from shears used without cleaning between animals, plus if the feet were trimmed so hard that they bled then infection could get in from the soil/dung. You can see why the routine trimming was done, to be as efficient as possible, but it's believed now that this causes more problems than it cures or prevents.
I don't think anyone else agrees with me (
) but I feel that the case is a little different with small flocks of relatively tame sheep. Here, we can see our stock all the time, and will notice if any animal is limping, or favouring a foot, so we have a look at the feet, and if there is overgrowth then we trim it off - not going too close, but enough to prevent soil being trapped under the overgrowth. Because we have only grass here, no rocks or hard standing, and because we live in a wet area, we find there is often a little of this overgrowth, and that it proceeds to rot only when there has been somewhere for soil to be trapped. So we wouldn't look at a foot and say - no redness between the cleats, no rotten smell, therefore we won't trim off this raggedy bit of hoof. We would trim it carefully.
You can't easily do that with 2000 sheep to get through in a day. I'm sure though that a good shepherd will catch up and look at the feet of any limping animal even in a large flock - that's what dogs and quads are for.
So basically what I'm saying is - use your common sense to interpret the advice