It is not true. Many people will say that swordtails, another species of livebearers, will turn from female to male if there are not enough males, but this is untrue. It is merely a late-developing male.
Depending upon where you purchase fish, always be careful when speaking to the salesperson. I often find that salespeople in chain pet stores (I won't name names but they all begin with Pet-) have no clue as to what they are talking about. If you really like guppies, go check out the American Livebearer Association (I don't know the link) or TFH has a pretty good softcover book on guppies, as well, called "Guppy Quarterly", I think.
Liggs