I finally managed to have an honest-to-goodness clearance issue. I've been playing with low-angle frogs in a couple of my smoothers, and had a 40-deg one in my LV custom 4-1/2 last week.

I swapped a new blade in while working some Red Alder with a medium cut (several mils, I didn't measure), and promptly had cutting issues. Swapping to a different, equally sharp blade with a lower bevel fixed them. When I went and looked at my spreadsheet (I know, laugh away) I discovered that I'd grabbed the wrong spare blade, and the one I'd used was set up with a 35 deg secondary bevel. Just for kicks I tried the problem blade on some Sugar Maple and Yellow Birch scraps (both about 1.5X as hard as the Alder) and didn't see issues.

So here's one empirical result for the endless clearance debate: 5 deg is low enough to start to cause trouble on a fairly soft hardwood (~1.3 mpsi modulus).

The reason I'm posting this is because I'm firmly of the opinion that extra clearance is nothing more than wasted blade life, so it pays to know where the real limits are. I think it's good to have some margin, so I'm going to continue to aim for ~10 deg clearance.