I usually hang out on the tandem forum, since the wife and I are buying a new Burley Rock-n-Roll mountain tandem this spring. We are getting a mountain tandem so we can ride both on and off road.
We are trying to decide if we want to get the drop handlebar option, or stick with the straight-bar and throw some good bar-ends on it.
We don't plan on riding any technical singletrack, it will mainly be dirt roads, fire-roads, and some two-tracks...nothing too hairy. That's what we have our singles for. We will mainly be doing centuries, light weekend touring (with a BOB), and maybe some longer supported tours.
Which would be the better option...a straight bar w/bar ends, or drop handlebars? We are looking forward to hearing your replies, so we can make the most edumacated decision possible!
Bruce Gordon Ti Rock N Road , Fat Chance Mountain Tandem 
I went from drop bars on our Santana road tandem to flat bars with bar ends on our Fat Chance Mtn Tandem after touring w.full panniers on the Santana on unexpected gravel roads. Control was the issue to be fixed. The flat bars have been great even on technical single track with panniers unexpectedly.