I've made lots of fencepost bows. All of them I made up in Nebraska were great successes, and I still have some of that post left to splice up some more staves with. My last attempt, my first in Texas, failed. Here is the thread:
http://www.primitivearcher.com/smf/index.php/topic,22983.0.html (I had to edit the posts to put the pictures back in after changing picture servers.)
I have never sealed a fencepost back when I got it to a growth ring and have never had one crack. But, all my fenceposts were VERY old. If your post was lying on the ground it could still have some moisture in it. It's never a bad idea to seal a bow back, but if your post was recently standing and old, it will be well seasoned, believe me. I think old fenceposts, especially old corner posts, are the best bow wood there is.
Good luck with yours.
George