I think that it's important to leave the WIP versions of a picture somewhere, as a testimony to evolution. And also I think it's not very friendly for the kind people who spent their time to write something for us and then we just go and delete it, when we don't need it anymore. I think it's not nice, but that's only my opinion.

But I also don't like to clutter the personal galleries with WIP's. The ideal would be for the personal galleries to have two subgroups: WIP's and finished images. The viewers could opt to browse one or the other. I know it's complicated and a lot to ask...

So, my "ideal" process would go like this:
a) The artist posts a WIP in the WIP gallery. This is added to the subgroup "WIP" in his personal gallery. Anyone can post comments, from inside or outside of the Critique group.
b) The artist creates a new thread in the Forum, asking for critique to the group, with a link to his WIP.
c) Maybe a 2nd WIP or a 3rd WIP come, all handled in the same manner, with updates of the thread in the forum.
d) Some time later, the artis posts the finished version in the normal gallery (Vue, Poser, whatever) and this goes into his personal gallery, subgroup of finished works
e) The artists updates his thread on the forum, telling the group of the finished version.

That's it! The hard part is the creation of subgroups in the personal gallery, as we would have to "convince" the site managers to have this rather big change in the concept. If this isn't possible, each artist could do as he/she pleases: he/she could keep or delete the WIP from his/her personal gallery.
Another option is to keep WIPs offsite and put links in the forum thread only, as Acadia suggested. This has the big disadvantage that only the Critique Group members will see it, but that's up to the artist, of course.