I have seen many people use X axis as the main axis of rotation. I’m not sure if it makes any practical difference? However what I did notice is that in Blender with inverse kinematics by default the Z axis X axis points towards the pole target. So if you choose Z axis you don’t have to 90 degrees rotate pole target angle on lower leg ik. The downside is that when mirroring bones, the orientation seems to also get mirrored and so you have to change pole angle to 180 degrees on the mirrored bone.
What do you guys think?