Which nut are we talking about? The nut between the fitting and the plate can be held with Proseal (that's what I did with my -6A) or with a piece riveted to the plate that prevents the nut from turning (as in the -10). Even if you have an anti-rotation piece, you will still use some sealant to prevent leaking around the union.
The B-nut, on the other hand, should not have anything in the threads ... but! Inside the tank there is no handy way to check it or to safety it. That's the purpose of the SB, which prevents it from turning without contaminating it with sealant. However, sealant has been used to secure the nut and is probably better than doing nothing.
