Hi, I am a BMW technician and yes you do need to bleed them , because of the slanted nature of the M52/M54 they suffer from cavitation in the cylinder head. If you fill the system, leave the cap off and repeatedly squeeze the top hose you should see most of the air come out of the header tank, the carry on as you described. Wouldn’t be the first to damage the head/ head gasket if you don’t.