ENG, it depends on a lot of things. The main one is when the tree was harvested. Trees cut down in winter time tend to have less moisture in the bark/cambium layer, and they tend to hold bark better.
I will typically remove any really loose pieces by hand, then scrub the bark with a wire brush to remove dirt and smaller loose pieces, then check it again for pieces which I suspect could have a weak bond. I will then take an ultrathin CA (super) glue and soak the cambium layer/bark with the glue to help strengthen the bond. I have yet to have any problems with bark coming off after finishing a project, but it is a weak spot, so depending on the application of the piece and its use, I will advise some people that natural edges may not be best suited for their intended use.
Some trees generally hold bark better, some don't. Oak is a challenge, mesquite is usually pretty good, and pecan seems to be hit and miss.
ragoo, do a google search for 'granberg chainsaw mill' that is what I use.