Best is to do it yourself. Second best is to watch very, very closely.
That one morning at -35 when you let them do it results in a tankful of avgas in a Mooney TBM.
Not checking for water after top up is just as risky. " could not have come from us, must have been in there and stirred up when we pumped in fresh fuel". Thanks, buddy, I didn't see the sign advertising free refresher on glider approaches with every fill-up.