I`ve had a decent size comfrey patch for years and like Charles cut just above ground level, (I don`t think it matters). I usually do 2 cuts per year, I feel any more is a bit too much for optimum production. Output from my first comfrey patch started to drop off after about 15 years so I had to move it, (I suppose that is to be expected). However; all it takes to re-establish a flourishing patch is to dig up some healthy sections of root and replant, (I like to see signs of fresh shoots on the transplants). Most of my crop is composted and the black liquid percolates down into the rest of the contents of the bin. Some leaves are used to produce liquid feed.
Cheers
Pete