My heart rate also tends to skyrocket when I stand on hills, especially on the tandem, less so on my single, if I don't think about controlling my motions. I try to be in a gear that's enough to support my weigh, but where I can still keep a fairly high cadence when I stand. I typically wait until I know I can crest the hill while standing, before I stand up. Although on some long climbs - Mount Diablo for example - standing is just a nice change of pace.

Veronica