Standing may not be as "efficient" as sitting and spinning, but it is indeed a good way to power over short hills or stretch your legs on longer climbs. Cycling, after all, is not always about efficiency. IMO, the ability to climb while standing is one hallmark of an accomplished tandem team. You can practice by using a big gear on flat roads... also a great way to take a "butt break" without slowing down.
Regards,
Dave