If re-fuelling then going flying, I sample the tanks straight away. Sure, any water probably hasn't had time to settle - but it's better than not checking at all, and it will also highlight other problems, for example if you've accidentally put jet fuel into your aircraft. And in one case a few months ago, I did find water in my fuel sample, although it may have dripped into the tester from somewhere other than the fuel tank since the aircraft was wet from rain.
If not flying straight away, then taxy back to the hangar and leave sampling until the pre-flight before the next flight. I always make sure I check the fuel strainer in this case, too, since anything that was near the fuel pickup line immediately after refuelling will by now have found its way to the strainer.
I also sample fuel on the first flight of the day (the A-check), and any time the aircraft has been left standing out in the rain.
FFF
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