Archery is about consistency. Line it up in the same place, but try not to concentrate on it, just briefly check it. IMHO, there is no place where it "should" be. I think the Koreans line it up on the edge of the sight ring. I line it on the left edge of the riser.
"if you have your stance and anchor correct, then you don't need a string picture"
Okay, to disprove this- get a good stance. Draw back, get a good anchor. Move your head slightly- the string alignment changes quite a bit for a little head rotation, but the stance and anchor can be the same.
"should be dead centre, otherwise your head is all wrong"
People are built differently. People shoot differently. You can't simply say your head position is wrong. Go back to them and ask -why- a particular head position is wrong
Even if your anchor and 'stance' is so good, you can't say no to another reference point. However, don't constantly worry that your string picture was different to the last shot or you get overly focussed on it and everything goes to pot.