I can't offer a lot on the accessory wire or the headlight bucket wire, I've never tested those. But those wires have to be close to battery voltage or they would be almost useless for anything but a light.
Have mine connected to the battery and I still see fluctuations with turn signals, high/low beam, etc. It's the same in almost any vehicle.
100% accuracy on voltage isn't so important as long as you can see that it's charging. For instance, you wouldn't want to leave home for a 100 mile trip if you see the voltage is lower than when the bike wasn't running. Or, you're riding along and see the voltage has dropped into a discharge reading, then you know it's time to head home instead of doing that extra 50 miles. Either wire (acc. or bucket) should be close enough to monitor the charging, unless we start a stator output contest.
I'd say hook it up where you want to, you can always change it if not happy. I like going to the battery, but if you want it switched by the key, one of the other two are the options you have. That inline fuse makes it nice for a direct battery connection.
One problem with switching through the key, you can't charge a phone or use a device without having the key on. You can probably charge phones for days before the battery would be dead.
Find your favorite hot wire, positive goes there. The ground can go anywhere there is a ground, a bolt almost anywhere will be grounded, unless it's threaded into plastic. Any additional switching, just needs one wire to pass though the switch, and that can be a hot or ground. Mine is switched on the ground side, it just makes one less wire that can get shorted out.