Want to go for a DAMN Ride? So do we!
It can really depend on many factors. Second might be a decent "rule of thumb" (at least till you get more experience and can guess better for yourself). Second is often low enough and you can use your clutch to bring your revs up as needed.
I will down shift when I need to on a big hill. I know many times in the past I would down shift too late and not use the clutch enough and stall after getting it into first.
Dirtygrunt had a good suggestion. The Aussie guy (Barry) who does the Crosstraining Enduro videos is great! His vids are well worth the time to watch and think about just before and after you go for a ride.
Hills are the main reason I've been riding big bore MX bikes forever. Momentum and a big fat midrange hit are my keys to hitting a big hill with no worries. 3rd gear @ the middle of the power so as it steepens you're only a twist away from mo power. Stand up for shock absorption and look ahead for obstacles and line alternatives.
On my DRZ 3rd was not working, had to downshift in the middle where there is smth like a step. For me it wasn't really hard, there was a great traction even after rainy night and with my worn rear tire, just had to keep myself balanced on those rocks. On 2nd gear it was easy, not even too exciting after a couple of runs, I liked better the shorter climb between Deer Pen and North South trail
Every day is a good day when you ride