Back in the day I raced on everything from 165 to 178. There's a difference, but 'm not sure it's important. If you go back a few decades, you see that crank length has been getting longer and longer. A fellow who would have been on 170s in 1960 would be on 175s today -- and might have passionately argued both, in each different time.
I have come to firmly believe that a 2.5 mm difference -- even if perceptible by an extremely sensitive rider -- is completely unimportant. As I write, my three LD bikes currently hanging in the garage have 165, 170, and 174 cranks. They all work just fine, and other differences between the bikes completely overwhelm any differences in crank length.
(Yes, I know I've mentioned a couple of really weird crank lengths. No typos, just an old pair of 178 mm BMX cranks on a track bike, and a pair of 174 mm cottered steel cranks on a four speed.)