I'll probably need an airport tour to answer this to my satisfaction, but I
thought it merited a discussion here anyway.

It seems pretty standard that, after an airplane is refueled, you wait 10-15
minutes and resample the fuel to check for contaminants (water). I recently
witnessed a situation where the fuel was checked during preflight, but not
after refueling. The reason given was that at this particular airport, the
fuel supply is replenished/recycled daily, if not more, and so there isn't
time for water, or perhaps *enough* water, to collect in the storage tanks
and end up in an aircraft's fuel tanks.

I'm not convinced. Work experience with mechanical things tells me that
mechanical things malfunction. Could water get into the fuel truck's tank?
Could water get into the storage tank? Could water get into the tanker
bringing fuel to the airport? Could there be water in the fuel at the depot?
Etc...

I really don't know enough to answer this. It seems that the most likely
causes of water contamination would be condensation in an airplane's fuel
tank, or a leak in the airport's fuel storage tank. If the fuel doesn't sit
in either for long, then that probably would reduce the risk to near zero. I
would imagine that there's some other failsafes in place to prevent water
from getting to the aircraft - perhaps the refueling trucks are individually
checked before they are sent out. Still, if water gets into the system at
ANY point, seems it's going to wind up in SOMEBODY's gas tank, no matter how
often the supply is recycled?

While we're at it, are these types of fuel checks standard procedure for ALL
aircraft, or just the little piston-powered ones?