My experience on the tandem with STI has been that yes under extreme pressure (17% grade) you can throw the chain when shifting and standing. However, we have shifted more times successfully while standing than not. Everytime we ride back home, we shift while standing up to climb our hill.

Riding the tandem by myself (a funny sight) I often shift while standing. It is the one advantage STI seems to have over friction shifters in my opinion.

Under heavy load bad things can happen whether sitting or standing. "Heavy load" is going to vary from person to person as well. It's a function of weight and power.

v.