I boot up most rides. Having a horse that wings, and is narrow behind, I want to keep the legs covered. Lazy rides I won't boot up, or if I'm just hacking and putzing around the property.

ALWAYS booted for jumping. Normally open fronts for the front legs and DSBs/closed fronts on the back. The only exception is hunter classes where boots are not allowed.
Bellboots if shod and doing heavier work (forward gaits, lateral work, jumping - anything that would encourage a clip)

I've seen more horses cut their legs open from being bootless than I have from horses who wear boots.

However, I have seen LOTS of injuries in the past from poor polo jobs. Poloing makes me paranoid.