depends on how i feel... no right or wrong way on a deck... if it's load bearing (roof on top) then i pour a footer, leave about 8" or more of exposed post, then pour a second layer around the post - to lock it in, of sorts...
sometimes i dig out, just use a solid 4" block, then pour all around it later... or on some, they spec a sonnen tube and plates if i'm doing a custom house...