Evidently, yes, it is a myth, even if it’s not 100% unfounded. Yes, there was some disapproval of the premiere from the upper balcony (which may have had more to do with social politics than with the work itself), but little more – and crucial evidence to that effect has been lying in plain sight the whole time since. The caused-riots-in-Paris!! bit seems to have come from an American effort to drum up excitement and sell tickets.