My personal preference -- at the beginning of a draft -- has always been to just go with whatever Yahoo has in the db (freezing it at some point for consistency). In the past, I'm pretty sure we haven't even been that restrictive. I'm almost certain that some major prospect (Heyward?) was not listed in Yahoo, but he was still drafted in Cafe leagues, with the understanding that his drafter would get him if he was added to Yahoo before the season starts.
But whatever the rule is, is fine, as long as it's clear from the start of the draft, which is why I brought the subject up early. So I'm voting no, even though it's quixotic.