InCopy CS5 or CS6 with client InCopy CS3?

Hello

I consider offering my clients to use InCopy to ease their workflow with me. I currently use CS5, my clients for some reason all work on 10.5.X and therefore could best use CS3, if I'm correct. Are CS3-Incopy documents compatible with those from CS5 or even 6 and vice versa? (I understand InCopy, unlike InDesign's Story Editor, works as a standalone application and saves its own files, right?)

Thanks a lot