This is interesting not only in a practical way but in a speculative way: to see how different people like to climb and to see the bike designs and builds that encourage their preferred climbing methods.
I've wondered about this when looking at the high, swept-back bars on so many Rivendell builds: it looks as if the resulting position more or less forces you to sit rather than stand, and therefore twiddle rather than mash -- and I've long noted the huge inner cogs on so many Rivendell builds. I recall the awkwardness of standing to climb when riding Indian roadsters; the geometry put the ends of the bars close to your knees, and since I often stood to climb -- single ~70" gear -- I always felt as if I were about to fall over the front wheel.
Of course, you'd not feel quite the same on Rivendells with the long top tubes and very long reachback bars, but all the same neither present the best position for agressive standing.
Me, I feel just the opposite of Brian: if I gear down and spin I quickly run out of breath, and I much prefer to slide back in the saddle, lean forward, and push the crank over TDC at slow rpm in a relatively high gear on gradual climbs, and to stand and mash when I need more power -- I often gear up a cog or so when I transition to a standing position. I suppose the difference is a matter both of individual physiology and habit, but I've been mashing and standing on climbs since I was a boy riding hills at 5K+ feet on my first "entirely by me" bike build age 15 that, because of my complete ignorance, had a single ~90" gear.