This is a large grey area and subject to individual interpretation and much debate.
If you post a completed piece on your own site an editor may or may not consider first rights to be used depending on your traffic. If you are posting it for critique on a well-trafficked message board I'd suggest labeling it as a work-in-progress. I wouldn't title it and I'd remove it after receiving my critique. That should cover your buns regarding first rights.
If you post it on a private, password-protected forum, very, very few editors would consider that published.
The main concern of the editors I've talked with about this is "how many people have seen the piece?" None cared if it was numbered in the dozens. For a couple the magic number was 500. My sampling was small though, about five editors.