91 Octane is what I use in my small engines because:
1. It normally doesn't have ethanol
2. My 372xp chainsaw requires atleast 89 octane
3. I don't have to worry about old 2 stroke mix (within reason)
4. It allows a bit of safety margin for an engine running to lean before I notice it, less chance of preignition.
5. The gas might end up in my YZ250
6. Gas is the cheapest part of using most small engines, how many engines have you used the purchase price in gas? Even in used equipment its hard to run $50 of gas through it, or $55 if you use premium.
I guess if all you use are 20hp/L Briggs then you don't have any real reason to get anything better than 87. I think good gas is just cheap insurance, plus I don't have to keep track of different octanes in my shed.
Ian