We're pretty new to tandeming, but we tried putting our pedals a little out of phase and it seemed like it helped us a bit with keeping the power steady during seated climbing. It also didn't seem to make our standing any less smooth. We didn't move the pedals very far out of phase, though, for fear of losing cornering clearance and making standing too awkward.
We currently have our pedals in phase. As we've become more experienced, we've gotten better at spinning at a higher RPM (which probably reduces the benefit of having out of phase pedals) and we haven't felt any need to move them out of phase (although I suppose we might try it again in the future). Our advice is to play around with pedal positions until you find the position you like the best.