I wouldn't do it yearly unless you spend all summer sitting on them. Just varnish the tops now and then...probably when you have the varnish out for something else.
Here's a seat out of that old Chestnut I'm restoring. Good condition with a unique frame from the early days of Chestnut canoes.
I guess they didn't varnish the bottoms back when either...Thanks for the reminder, this thread is very timely for me, I have a couple of seats for another Bob's to do before spring.