This is a good topic. I've never not flown for finding water in the sump-cup. I just keep taking samples until there is no water. Of course, if I had to draw a ridiculous amount of water out.. I'd want to know why before flying.

It depends too, on how much fuel is in the tank. A mostly empty tank can have you drawing quite a bit out at the sump, and even after you get a clean sample, there can still be lots of water condensed on the tank's inner walls. If you're plannning on taking a weight sensitive flight, and topping off the tanks is not an option; then even "shaking" won't get all the water into the fuel and out the sump (unles you REALLY rock the wings and let it settle for a LONG time).

The biggest danger is not from having "some" water in the fuel (there will always be some), but from a concentrated amount working it's way to carb/throttle-body and starving the engine. Also.. when any part of the fuel lines have water only in them (like when that stuff in the bottom that you forgot to drain starts making its way through), it can freeze, if it's cold enough.

If a plane has sat for a long period of time... and nobody had the foresight to top the tanks off, you had best top them off yourself.. let the water settle and then drain it off. If it's that weight sensitive flight... then either go up and burn some of the fuel off.. or off-load it.

The biggest question about your lineman story would be... How full were the tanks ?  Or did he top them off ?

Yeah, my buddy said the tanks were topped off about two weeks ago, right after its last flight.  It was a 182 so there was still a lot of fuel in those tanks... but still, that's nearly a gallon of water from a pretty short period of time.  Our school has a couple of planes in maintenance that have been out in the weather for months and only have a few drops of water in them.