I go with the guys saying 'don't do it'. I completely fenced 2 acres with ties used as both corner and line. Didn't have to cut any of them but did a lot of cutting trimming braces, terrace ties, etc over the years. I figured one cut with a sharp chain and then chew through whatever cuts are left. The problem is not, as has been pointed out, the irons or spikes, it is the embedded gravel which cannotn be cleaned out. The creosote also gums up things. I would not ever cut a tie for someone else.
Harry K