From FCOM 2.01.30 P10a
"APU starts or shutdowns are permitted during refuel/defuel procedures. If it is necessary to operate the APU, the limits that follow apply :
An APU start is not permitted during a refuel/defuel procedure if the APU has failed to start or an automatic shutdown has occured
A normal APU shutdown must be completed if a fuel spill has occured during the refuel defuel procedure."
That's all very well, but the fuel companies have their own rules. It is good manners to ask the refueller whether he minds or not.
Cheers,
mcdhu