As a farmer, I am sceptical of foot trimmers who charge by the foot, as this gives them an incentive to trim regardless of need. So we do our own feet here having been trained by our vet.
Admittedly we have British Friesian cows, who are regarded as generally better on their feet, but very few actually need routine trimming, a check maybe and all cows are done at 200 days in milk with very few actually needing much trimming at all. We get very little lameness and I put this down to the infrastructure and the way the cows are managed on a low input system. Feed is pushed up regularly to ensure that cows can reach it without straining which reduces intakes and puts pressure on front feet. The feed passage is also raised 6 inches and the parlour exit is large to eliminate sharp turns.
We have had 1 lame cow in the last 2 months - she had an infection in the white line on a back foot, but I wouldn't say routine trimming would eliminate all lameness. Can't remember the last cow that was sent as a casualty for lameness.