It is a personal preference thing but it can make a big difference to how smoothly you ride. They often put longer cranks on larger frames as they expect these will be ridden by taller riders, the notion is that the longer your legs are the longer you will want your cranks to be. Even on larger frames it seems 170/175's are the norm.
I am 6'7" and have had cranks ranging from 165 to 180 on my commuter bikes over the past few years and have determined that I like 175's the best after putting in about 8,000km per year. That 5mm change on either side can have quite an impact on the range your knee moves through. It also depends on what sort of riding you are doing, the 180's felt good on my mountain bike but on my commuter they started to gradually hurt the front of my knee.

My son races BMX, it is really obvious when the crank length is wrong with little kids on BMX race bikes. When the cranks are too short they can't get the bike moving out of the gate unless the gearing is really easy, which means they really have to spin to get any speed on the straights. When the cranks are too long the pedal stroke becomes really choppy. It becomes a bit of a balancing act between the gearing and crank length.