I'm strong on hills, but rarely get out of my seat - and I'm on a hybrid, and pretty positive there's a correlation there. I HATE the total change in balance; it just doesn't feel right at all.
It pays to shift early, and the other thing that really helps me is making sure to keep my stroke steadily going 'round and 'round the whole big circle.
I got stronger doing intervals without standing up. THere's a little excuse for a hill near me (there are very, very few hills around here) and doing repeats up that sucker does a lot for the endurance and strengtha nd speed.