I've always believed that it's better to have a gear and not need it than to need it and not have it, so I've always used a triple, with the smallest available granny ring (24T mainly, but 20T on the current bike).
Having said that, sometimes you do just get in a rhythm where a bigger gear seems right - on one Spanish trip day 2 was over the Veleta to Capilera, which I think was about 37 km at 6 - 6.5%, and I never got off the middle ring (38x32). The previous day though, we'd been right to the top by way of a warm up, and I had needed the granny (24x32), both for the rough bit at the top, and for the climb up from Guejar Sierra (
the small road, not the one the Vuelta has used)