when the sump being at the low point is irrelevant
It's the combination of the leaking caps and wrinkles in the bottom of the bladder tanks that lead to the term "killer caps".
You can check at the sump for contamination all you like, but if there is water trapped at the bottom of the tank by a wrinkle, it is not going to come out...until (e.g.) rotation!
Please see below, an extract of a letter from Cessna Pilots Association. It's specific to c182 but I would assume relevant to all aircraft with bladders prone to wrinkling and leaking fuel caps.
FUEL BLADDERS AND FUEL CAPS
If you fly a 1956 182 thru a 1978 182Q, your airplane uses rubber bladder tanks in each wing. These bladders have a tendency to develop wrinkles along the bottom. The wrinkles act as little dams that can prevent water from moving to the sump drain. You can sump the tanks at pre-flight and see no water, yet water could still be present in your fuel tanks.
To make matters worse, Cessna originally installed flush-style fuel caps on these aircraft. The caps can leak if the aircraft is exposed to moisture. If your fuel caps have a small hinged pull-up handle that fits into a recess in the cap, you have the dangerous fuel caps. At CPA, we call them "killer caps."
There have been a number of engine failures immediately after take-off even though the pilot sumped the tanks thoroughly during pre-flight. Some of these incidents have been fatal. The FAA issued Airworthiness Directive AD 84-10-01 to deal with the problem. It requires inspection of the bladders for wrinkles, and suggests changing the flush-style fuel caps to umbrella-style caps.
If you fly a bladder-equipped 182 that still has flush-style fuel caps, the Cessna Pilots Association strongly urges you to change immediately to either the Cessna umbrella cap (kit SK-182-85 available through any Cessna service center) or the Monarch Development cap sold by Hartwig Aircraft Fuel Cell Repair (phone 800/843-8033 US or 800/665-0236 CAN).
I beg you not to overlook this fuel cap situation on your 182. Accident statistics show that the 182 Skylane has one of the highest rates of accidents caused by fuel contamination and the flush style fuel caps coupled with the bladder fuel tanks are the main cause.