My knees are fine, but my ankles have very little movement in them and almost no strength (due to tallipies as a child, hence stick legs), so I use 170mm on and off road because 175's make me turn a circle bigger than I naturally want to, therefore gicing me a kind of biopacey feel.
I only found this out by accident as my MTB's were always 175 and road bikes 170 as standard and I found the pedalling action on my road bike much more comfortable. So went 170 across the board.
If you can try a few lengths and see what is best for you, I am glad I did!
Somerset, like 'Heartbeat' with newer cars!
Retro MTB: 1992 Cannondale 3.0 Deore XT M730. 1992 Marin Pine Mountain Deore DX M650.
Retro road: 1992 Eddy Merckx Corsa Extra Telekom Shimano 600 STI. 1993 Dyna Tech Ti.
Modern: 2006 Wilier scandium Ultegra 6600. 2014 Wilier GTR Ultegra 6800.