I like the 170's on my road bikes, where I tend to spin at a higher cadence. Mountain bike and winter bike have 175's and now for the first time,180's on the 26 inch BMX. But my legs are fairly long with a 32" inseam. Yes, some people say that running too long of a crank can lead to knee pain for them. I feel that changing it up with different lengths is a good thing for maintaining flexibility and muscle stretch. It seems to me that as long as I don't push too hard or too long, no length is likely to cause any issues for me. Switching it up may even be a good thing.
Crank length does have a small effect on the gears I choose though.