Airbus explained it in a document that the APU bleed is automatic and it's OK to put bleed ON even before start is complete as it won't open unless there is pressure as FlyingStone rightly said, but you may not do it during cockpit prep because you need to check that there is no external air is connected as it is forbidden to use both together. You don't do it after landing after starting the APU either because it will suck kerosene fumes. System wise there nothing wrong. APU will create smoke in the cabin only if during last shut down the batteries were switched off before APU flap had closed which disrupts the de oil sequence.