Standard veterinary advice now recommends not routinely trimming off the excess horn.
Trimming routinely can cause the sole/wall to be shallow, predisposing to bruising and excess trimming can even cause growths called granulomas in the toes if cut too short.
The powers that be also advise for serious lamenesses such as footrot, to give antibiotic injection and spray with pain relief, and to trim a couple of days later to remove the dead horn once the animal is more comfortable.
I would tend to turn over any sheep that are lame, as soon as you spot them, to find out what is going on. If it is a deformity/serious overgrowth, then trim and leave, if potential infectious cause eg. scald or footrot, use blue spray, or add in injectable antibiotic if severe, then if still lame in a couple of days, check again, and if need be remove dead horn.
The most important thing is to treat any lame sheep as soon as you see them, because they are less likely to spread anything around, rather than doing all of them at once when gathering routinely.
Hope that helps
Suzanne