It used to be, in an older airline and older a/c, that use of the toilets was forbidden during re-fueling due to activating the flush motor. Now, in a more modern airline and more modern a/c with suction systems (still activated by an electric motor) Boeing OK'd their use during fuelling. No reason, just OK'd it. Could this be the same 'old wives tale' as the use of personal devices and mobiles etc? The rules are now much relaxed. I see the dispatcher walking near the bowser radio in hand and talking. I see ground staff using mobile phones all over the aprons. I know nothing of output power of isolation systems etc. it's just observation.
It clouds the truth.