Final fact of the matter is there's no "safe" time to have unprotected sex if you're looking to not get pregnant and anything else is superstition, whether or not you surround it in the trappings of fact. You're talking about a difference in probability, not a "safe" and "not safe". A few rare people have gotten pregnant after having sex while on their periods (I think that's kind of icky, personally but that's a different subject :D). The chance may be one in a million, but guess what, that means that out of every million people to do it, one person will get pregnant on average, and you're just as likely as anyone else to be that one lucky person.