I do both. It's not so much for added seal, but to hold the valley pan in place while installing the front and rear clamps and lowering the manifold in place. It takes very little but a very thin coating keeps things in place very well.
Whoever has this classic before me put a lot on it. Way too much IMO. Also hadn't cleaned any of the oil that leaked from the valve cover gaskets. I spent as much time cleaning as dismantling. It was filthy. Ton of silicone on the old valve cover gaskets too.
I use permatex form-a-gasket #2. It's non-hardening and easy to clean up with brake cleaner. I push a glob into the corners where the head over hangs the block. I also smear a thin layer on the head and manifold where the coolant channels are. I do not coat the entire edge of the block, just the corners and the coolant channels, and definitely not on the valve cover gasket. Remember, "less is more".
Silicone likes to break apart and find it's way to places it shouldn't be.