If your rings are as thick as you say, you should be able to sand out nicks and dings and still be in the safe zone. I know I always end up with stuff that needs a little TLC before tillering and I have had one mulberry or osage stave let go on me. And that one actually had a serious seperation between the growth rings when it failed so I think the tree had been subjected to some serious damage prior to me getting the stave.
Doesn't mean, however, that you can go at ring chasing like a blind axe wielding mass murderer and think you just need to use a little extra sandpaper to salvage the work. If you are concerned, just use the draw knife to remove the hard late wood and scrape the crumbly, crusty early wood with a scraper or knife blade.
good luck