All The Six Fifty Bees Thread! (650b, 27.5 etc.).

Internet charts are very conservative.
From my own experience in rims and tires, here's what I noted.

1. if the tire width is really close - or less than the the rim outer width, the sidewalls are more exposed to cut, scratches;
   
2. the rim is a bit more exposed - a heavy hit hit like a pothole can ding the rim a little, more worrying than a pinch-flat
   
3. can be dangerous when take fast turns (their is a risk you ride "out" of the thread and slighlty on the casing, resulting a sudden loss of grip)
   
   

I currently ride a 32mm tire on 28mm outer-width rims (a few years ago, these rims would be considered MTB/Enduro only), and it works fine.