Regardless of all the jokes and criticism, "hand reaming" to set final headspace was a fairly common practice some years ago. It probably does not have a place in fitting barrels for Benchrest guns, but it can and is done successfully by many old school smiths. My take on it is that with older style factory actions with less than perfectly straight threads and general sloppiness it can be difficult to gage the final headspace while the barrel is still set up in the lathe as there is a tendency for the barrel to "pull up" slightly when the barrel vise and action wrench are applied. It is not difficult to chase a few thousandths to set or correct final headspace if you have a good T handle adapter for the reamer. It helps to have some skill with hand reaming which is pretty much a lost art these days.
To answer your question, if you are fitting and chambering the barrel in a lathe yourself, there is no reason to leave more than a few thousandths for hand reaming, and it is probably not necessary to leave any.