One advantage to pouring dry is, elminating the mixing process while
setting posts, and less clean-up , but..............

I've always had water near-by, either with a hose or bucket, because it
just didn't seem right to work with 'crete without it.
I also thought it depends on the dryness of the ground.

what I've always done was placed 'bout an inch or so of water in the
hole.....then dumped the 'crete to about quarter/way, an worked it with
a diggin iron or wrecking bar..then plumb the post, add a little more
H2O...'crete...work it..plumb....
H2O...'crete...work it..plumb....
H2O...'crete...work it..plumb....
till you reach the top.

When you near the top, make sure you have the 'crete pasty, 'cause ya
need to end up with a slight rise/mound to allow the rain/water to run
away from the post....even if they are vinyl...besides it looks better.

IMHO......Sky