I've always run 175s because, well, that's what the bikes came with from the factory. When I replaced the factory 48/34 crankset on my touring bike with a 39/26 unit, though, I also opted for 180mm crankarms. That extra 5mm per arm translates to a 10mm (4/10") larger circle by the pedals, and basic physics tells us that the longer arms provide additional torque. Unloaded and/or on flat ground the difference is so slight as to not be noticed, but when loaded and/or climbing, that additional leverage adds incremental torque.