If it's true then Wobbles woulda been dead about 8 billion times over by now. The guy trips over his own two webs and winds up on his back like a turtle! You don't get more upside-down than that! If you mean like, holding them vertically, bill-down, it's not fatal by itself but it's definitely not a good idea to hold them like that because gravity + stress could cause whatever they last ate to come back up through their esophagus and then they risk inhaling it, which could asphyxiate the bird or lead to pneumonia. So all in all, just don't do it.