It's a continuum of personal decisions.
As others have said... it's probably best to get it running and just drive for some months before deciding all these things:
Do you want to drive regularly on wet or salted roads?
Can you commit to washing the salt off quickly when you do?
Can you keep it inside?
Do you care if it's going to rust a bit more?
Do you want to preserve for future generations?
As a museum piece, or with functional upgrades?
Are you going to enjoy the extra challenge of cleaning/painting?
Leaf springs probably thick enough to last with minimal protection... but other stuff would need to be really well cleaned before applying protection.
If you're going all the way to frame off resto (careful it's a LOT of work), I like the idea of doing nothing to the outer body, but then mega cleaning frame/underbody for SPI epoxy as an undercoat.
