I've always tended to attempt most climbs seated with the caveat that it feels relatively efficient. To my mind that's, at least partially, the key. As I begin to tire/struggle on a climb I change up a gear and stand on the peddles, once I'm going again I sit and go back through the gears. I tend to use the gear below the hardest I can push generally. When I was younger I guess staying seated was a 'point of honour' to me and my cycling mates. There is also the traction issue, and positional efficiency i.e. getting over the back wheel so you're weight is over it more. I think you might assume that standing puts you over the wheel more but I don't think it does, also it un-weights the front. To a degree this is more important when on the mtb, but I have, on occasion, lost traction on climbs when stood.