basically technically , during a refuelling , whatever the Aircraft type is , Boeing or Airbus , everything must be off , it is a question of safety ...
i do not see why the fuel pumps should be on , the fuel pumps are low pressure pumps and are just needed to feed the high pressure pumps ...
now , by experience , i have seen few times , where , on some Airbuses (old 340) , the total fuel qty was varying (until 500 kgs) , between pumps off and then on , the main reason being filling in the collector cell ... i guess it is the reason why , depending on the company Policy , some are asking to put the pumps on ...
as a tech , i am leaving the pumps off , then , when the refuelling is completed , i am switching the pumps to on , and waiting around 10 mins to see if the total qty is dropping down or not , just my 2 cents here ...
anyway , leaving the pumps on ON is disturbing Nothing during a refuelling ...
refuellings are managed by the FCMCs , depending on the qty required , the FCMCs will open and shut the refuelling valves , maintain balancing , etc ...
in a perfect world , Nothing special is supposed to happen , except if some valves are having internal leaks , if there are no leaks in the tanks , etc ...
normally , until the FCMC is not stopping the refuelling , Nothing is supposed to be done , but , thats the duty of the person in charge of the refuelling ...