I was told no need for a full bladder for 20 week scan. In fact, there were even signs all over the waiting room saying that. Besides if baby gets into an awkward position, they will do quite a lot of pressing which could be awkward with a full bladder!
Babies are so small they are still doing a fair amount of moving at 20 weeks. My one started the scan with head buried very low down, so sonographer was a bit worried she wouldn't be able to get a good look at head but then moved up to a transverse position halfway through.
And Madwife, like you said, my midwife couldn't tell the difference between head and bum at 38 weeks last time, hence we discovered baby was breech when I was 5cm dilated and in labour. The funny bit was she had a student midwife with her at my 38 week appnt - who she kept saying, there's no mistaking that - that's baby's bum etc. DS was 9'2 and 59 cm long when he was born - there is no way that baby turned at the last minute without me feeling it!