Thinking about putting up around 68' of privacy fence. Going to use metal posts and western red cedar pickets. As far as the rails/stringers go, is it necessary to stagger the joints? i was just going to use 8ft rails, 3 rails on a 6ft fence, and have the post set on 8ft centers, but after looking around i saw to use 16' rails so each post doesnt have 3 joints on it. im havng trouble finding any 16ft 2x4 in the red cedar in my area, so i would have to use regular pressure treated on the rails. So will not staggering the joints make it any weaker?

ETA: posts will be 8' x 2" with 2' in the ground set in concrete.

It will be fine, 2x4x16 cedar will be twisted up like pretzel no matter were you get it. What type of fastener are you using for the wood to metal connection?

8' x 2" posts?  I have only seen and done 4" posts

4" metal posts? Holy Smokes. You sure you aren't thinking 4"x4"x8' cedar posts? 2" is all I've ever seen or needed in a metal post.

Yup I was thinking wood.