I think it's "best practice" (there are those dang words again
) to have the 4 prongs visible, but there's no harm in having them facing the car seat. When I teach a class and parents, I teach them to feel with their fingers. You'll be able to feel if a prong isn't on the belt.
FWIW, I think the new manual contradicts itself on the locking clip. It says it doesn't matter which way the locking clip goes on the belt, but oh, you need to have the 4 prongs visible. Then on the slide, it shows it with the 4 prongs facing the seat. It's clear as mud! I wrote someone in the certification program at Safe Kids to double-check and bring the confusion to their attention (though how it could have slipped by, I don't know--when I was prepping the class the first time I caught it).
Anywho, this was an issue for me years ago and I asked around then and the consensus was there is no right or wrong way to put a locking clip on a seat. Just get the darn thing on and it'll hold tight regardless of which way it is facing.