There aren't strict guidelines, but if something isn't much upvoted and/or doesn't seem very important, I'll move it to Discussion. Trying to post to Main is not a crime. On the other hand, moving things back from Discussion to Main after an editor moves them is a crime.
What about the reverse? Moving from discussion to Main once the author notices that not only his introspective evidence says the text is good, but also others?
Yesterday, someone moved one of my posts from Main to Discussion without telling me. Again.
I encourage the site administrators to show some basic courtesy to the posters who provide the content for the site. I believe this would be a better way of doing things:
1. Have a policy on what has to happen to move a post from Main to Discussion. Who can do it? How many admins are there who can do this? State this policy in a FAQ.
2. When you move a post from Main to Discussion, make a comment on the post saying you have done so and why you have done so.