As transilvana says the poor crew wouldn't have had time to go the toilet let alone supervise fuelling!
There is a requirement for the Captain to nominate a "Refuelling Supervisor" which has to be a suitably qualified person and will generally be the First Officer, the Captain (himself) but could be an Engineer or another crew member who is travelling "jump seat". The Refuelling Supervisor is required to establish communication with the flight deck and report any problems asap. The RFS is, as far as I can recall, only required to be "on station" when pax are on board and/or deplaning or boarding.
In my 5 years with the said Company I ensured this requirement was strictly complied with and am sure that my colleagues did likewise.