The last three versions were Flash 4, Flash 5 & Flash MX, right? I'd tend to stick with Flash 4 features, myself. Very similar to Acrobat: You get the plugin somewhere and then totally forget about it... until you're told to upgrade by a site.
Is your Flash content presented as necessary enough to justify the time spent to download? Do you want to take the risk?
Thanks all. That gives us exactly what we need...especially the Macromedia link in Korkus' post.
So, it looks like Flash 5 features are relatively safe. However, this team wanted to go with Flash 6, and the content they are developing is important to their business mission. Looks like they'll need to accept a limitation.