Free with no play on BB, and solid wheel axles. With QR axles, you need to have the very slightest play because the action of closing the QR tightens the preload on the bearings. Practise on your wheels. If you still have play after fitting them to the bike and doing up the QR, they were too loose.

With headsets, the test is more whether there is "indexing" or a notchiness when the handlebars are turned, or whether there is movement felt when you apply the front brake. To test the latter, put on the brake, wrap your free hand around the top headset cup and rock the bike back and forth... if there is looseness, you will feel it in that hand. Ideal adjustment is in between those two tactile measurements.

Jobst talks from a very engineering perspective, and IIRC, his preload suggestions aim to get the balls or rollers doing what they are supposed to do from the get-go. But it does become a balancing act, and to me, accurately measuring preload is more trouble that it's worth.