I thought everybody booked aisle and window and left the middle unbooked. If you get lucky, you have an extra seat; if not, the middle seat will almost always be willing to swap for one of the other two and you can still sit together.
Nope. If an empty row is available, book the middle seat. No one wants to sit next to the weirdo who chooses the middle seat first.
But then if the plane is full you are stuck in a middle seat whereas, like gp pointed out, choosing window + aisle for a two-person booking almost always lets you fallback to window + middle or middle + aisle if the middle seat does get filled so there is really no risk in taking the chance.
I do this but I’ve been told other’s views on it range from “seems fine” to “this makes you a terrible person”