We've never taken advantage of the opportunity to start with a light fuel load and top up half way, but given that the stop is 5 minutes including the fuelling, I sometimes wonder if we are missing out. If you want to get your 5 minutes exact, it will require someone to be close to the refuelling point to start timing the stop. At some circuits that is easy, others not quite so.
If you have a car problem, the 5 minutes can allow time for a fix- it's helped us once this year! If we had been refuelling as well we would have lost time.....