I've ridden everything from 185mm to 155mm cranks on different bikes for different reasons, and I have found that most riders who swap out their 170mm cranks for 175mm (or any other exchange) generally only have pain or discomfort because they failed to make the other fit adjustments that the new crank required.

These days my SS 29er mtb uses 177.5mm cranks because I use it mostly in tight technical riding and I like the extra leverage, my 26" mtb uses 175mm because I use it for all-around off-roading, my commuter mtb uses 170mm because I only use it exclusively on-road and I like to spin on it, one of my road bikes uses 170mm because I use it in group rides and the seem to work best for me and the other uses 172.5mm because it's more of a layed back distance ride, and my FG uses 165mm so I can spin like a madman on the downhills, and I barely notice a difference between them in actual use because I ride each bike differently.

As stated by some others here, I don't believe there is an "optimal length" for any rider or any type of riding, rather I believe that fitting the rest of the bike properly and buying the crankset that best suits your desired pedal speed/power output needs is more important. It is a matter of finding your sweet spot, but on each bike for the way you ride it, and getting the overall fit right regardless of what length crank you choose is more important than the crankarm length itself.