After getting the ever loving snot dropped out of me by a buddy on a single speed mountain bike up climbs in Western North Carolina, I'm inclined to say use a bigger gear. He was running 34x21, so even cranking out a slow grind at 60rpm he was doing 8.4mph.

If I'd little ring it I'd be going 5-6mph at 90rpm, and if the bailout is there and I'm already having a tough day, I'm inclined to use it. If all you've got is 39-27, then that's all you've got. I'm generally opposed to this sort of non logical thinking, but there is something to be said for not having too much of a bailout. It keeps you pushing.