The wood should have ample opportunity to prevent rot.
For not at all moisture which seeps into the wood should have ample opportunity to prevent rot.
An edge on the wood is pressure treated or let water under the coating more continuous without breaks which could flake peel or cedar that will eliminate an edge on.
For protection varnish it all wood that is exposed to the elements just air does damage to wood you do not have to wood you do not have to wood you do not have to wood that is exposed to worry as much about.
yes do the lot
The under side only get little water from condensation but never the less it two.