I always thought a psu should have a ground cable since I dont see how they ensure it otherwise.
Any screw that also touches the case is fine. My drive screws dont because of measures I took against vibration so I'd use the motherboard to case screw probably
Been a very long while since I looked at the tagan manual, but I remember readig something along the lines of the Ground cable should be conected to a motherboard screw too help against voltage ripple, or something along those lines. My tagan is the very first revision with the ground connector more like a hook which is a pain to screw down , unlike the newer ones with a screwhole.