The reason the bones twist is that the X axis of the bone with the IK constraint points towards the pole target. Since most people use the X axis as the rotation point you have to adjust the pole angle which will be based on the bones roll which is usually 0 or 180. Thus this determines if you use 90 or -90. If you adjust the roll so that z is the main rotation the x will already be facing the pole target and there won’t be any twist, but animators will hate you as a rigger 
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Now you mention it, it makes sense up and down the bone should be Z, up and down direction in the standard scene.
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@NP5 I can’t take credit for it. I’m watching a few videos on rigging. Including Grant’s new course. I can’t remember if it’s Humane Rigging or DanPro on Youtube that pointed this out. The Youtube ones use old versions of blender. So, Grant’s is great for seeing how to apply it in 3.x+ Blender interface.
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