Concrete has a corrosive way of destroying wood. If you wrap the posts with a builder's paper (tar paper) first, you should be fine. Put rock in the bottom of the hole and adjust the depth with more or less rock. The rock will leave the bottom open to drain any moisture and is easy to work with. When you get the height right and the post aligned, then pour concrete in up about a foot, final adjustment, tamp, fill the hole in, tamp, top off the hole, tamp again. Recheck your plumb and alignment, move on.
Just be careful to not rip the paper while tamping. Trim off the excess paper when the post is set.