Only you can answer the question about which player version to target when you publish. Many factors come in to play.
- Is your audience early adopters and likely to have the latest player?
- Is your content so insanely great that your audience will install the latest player just to see your content?
- Is your audience likely to be corporate users who have a policy of being 2 (or 3!) versions behind the latest and greatest?
- Does your content require some fancy feature only available to plugin X or greater?
And so forth. So once you have answered those questions you can say, "Yes, we must publish to Flash 9 and nuts to those who won't come up to at least that point." (or whatever you decided.)
After that you should add detection code to your html page that will redirect the user to some alternate content if they don't meet the minimal requirements. You can use the basic code that Flash generates to do that or you can use swfobject to roll your own.
You should customize the alternate content to make it clear what is needed and how to get it. Maybe even give a little preview screenshot or something of your content to tease them and convince them on how much they are really missing out on, etc.
Also in the past there were some bad detection scripts out there that would sometimes deny the user the ability to get to the content based on errors of judgement. So I generally like to add a link that says something like, "Yes, I really do know what my computer has installed and your detection script is stupid. Take me directly to the content. (Warning, if your computer really isn't up to date you will see only a broken link.)"
