8" hole is correct if you backfill with soil. but some fence posts should be set in concrete. for fence post you need to put the end post (end of the line) or corner post into concrete and every 3rd post into concrete. concrete is cheap, so don't worry. for 6x6 post you need hole diameter of 11", but dig 12" to give wiggle room for fence alignment. also for posts that sit on either side of a gate should be in concrete.
"if you can't be handsome, at least you can be handy" -Redgreen