It’s a matter of preference. I personnally prefer rigify’s X down instead of Z down, because I like to key main bending on the X axis. In this case the Z axis is the main bend for the elbow and X is the main bend for the knee. As for the direction, animators tend to favor working with the charater’s left and using Flip pose to copy to the other side. By having axis pointing downward(arm)/Backwards(Legs) this makes it positive angles. If you flip it, it will be negative angles. Also when you symmetrize the roll gets flipped. Positive becomes negative and negative becomes positive.
