Personally we prefer this approach to rigid 'rules'. Although we have a 'desire' that we both know each other's partners in advance to just get second opinions if you will, we also acknowledge that we know & trust each other well enough to let things develop as they naturally will which means some 'activity' might occur before we all get to meet - at least in person.
Actually GS our rules are relatively simple (and the only one that does not have exception is to practice safe sex). In terms of notification... prior is preferable, but in cases where the individual is likely to never be seen or heard from again, post can be ok.
I have also really pressed my husband on how he would feel if I developed an emotional bond to some one else, and he is pretty convinced that as long as the new bond remained a 'secondary' one he would be ok.
Thanks to all who are posting!