hey buddy
it depends on where u live as far as rust goes....in canada we put alot of salt on the roads so rust is a real danger.
i have a little on some of my 03 bolts in the underbody.
however i am also getting my car "rust protection"
it is a process where a lubracant is sprayed on the entire under body as well as INSIDE the body pannels.
this helps to seperate moisture and protect from salt....i have had it done on my other car before my 4 runner and my folks had it done to theirs and beside a little rust on the under body none any where else.
not too sure on what toyota says.....i know their warrenty for rust kinda goes out the door in canada
cheers