Hardiness zones are based on average minimum low winter temperatures over an extended period of time - usually at least 30 years. 10F is the lower limit for zone 7b........that's quite a stretch from your reported zone 5/6. Even in my zone 8b/9a, we can get an odd cold snap - 2 years ago it hit a record low of 12F in early November! Typically it is recommended that anyone in zone 6 or lower offer winter protection on their bigleaf hydrangeas.

And while cold winter weather can certainly be a problem with bud damage, it is more often early freezes (before the plant is fully dormant) or late freezes (as the plant is emerging from dormancy) that cause most of the damage.

All Summer Beauty is a 'reblooming' hydrangea, meaning it will bloom on both old and new growth. Typically there is less concern about bud damage on remontant hydrangeas as even if bud damage on old growth occurs from a cold winter, you can still get flowers from the new growth. But if you want to maximize the flowering potential, winter protection is recommended for reblooming hydrangeas as well.

Years ago we got temps of 15 below zero...
I remember 2 weeks straight of it from time to time.
Now our climate is a lot more mild... (Past 10+ years)

I'm going to start another post on when I should start
winter-protecting...Thanks for your wisdom gardengal!