I bleed my radiators when it is running from each radiator. You have a
pressure gauge on your boiler you should rely on as autofill valves are
not always perfect. Also filling should be done on a cold boiler not
hot. If you bleed with the unit off and cold nothing may happen , or
little compared to when it is hot and under higher pressure. I have
found my autofill to over fill by 3lb.