I think it is common practice NOT to Sample the tanks after fueling, also NOT to take fuel from a field's supply after that supply has been re-stocked.
This is because, when transferring fuel, it gets all stirred up in the process. So you need to wait for about an hour for the water and contamination to settle, before taking a sample. Most small planes are sampled first thing in the morning, after an overnight stop, in the 'A' test.