Timber was clipped when I got him, and I haven't clipped him since (7 years now). He doesn't fly though, I don't think he ever fledged. That said, he doesn't pluck. I think it depends a lot on the individual bird, but as SRSeedBurners said, I'd assume they are less likely to pluck if flighted but I have seen members with flighted birds who did pluck.
Ha, this post really doesn't add anything but that's the only experience I have!