"A P6 prop will still function as expected in earlier versions, right?" That depends on what your expectations are. :) The fancy material shaders won't work, of course. If a Poser 4 user opens a prop that was saved from Poser 6, they'll see a message telling them that it has a higher version number, and the only option is to open it anyway. While that's not a problem, it's cleaner if your props open in all versions without it; and if you aren't using any material room shaders, I think a Poser 4 saved version of the prop would be a smaller file than the equivalent in P6.