I have 170 cranks on my trail MTB, and HAD 175 on my XC bike. I just put a ROTOR power meter on my XC bike, and decided to try 165 (wanted to go extreme to see if I could tell a difference).
I like the 165s on my XC bike. I feel like it is easier to spin faster. I was struggling with maintaining a cadence around 90, but now it does seem easier (I was at about 80, on average). I haven’t felt like my ratcheting or power move stomps have suffered at all. No knee pain or anything, for what that’s worth.