I think the bigger issue is that watching them the wrong way would ruin BCS. It’s an absolute treat after BB ending as you sort of know what is coming but the details are slowly revealed.
Agreed, but a bit less politely, Better Call Saul would be a frustratingly glacial drama about characters you don’t know or care about, dragged out for about 35 hours before it starts to get interesting, or indeed the titular character turns up.
Imagine going into to it without any knowledge of BB, wondering what or who ‘Saul’ was and why anyone should call them, only to see a strange drama about a low-level Lawyer called Jimmy/James bookended by an even stranger drama in black and white.
No, I don’t think anyone should watch BCS first.