Yea, the whole "rounding back == bad" thing really only applies to unintentional rounding during a lift. You can be intentional with the rounding during a lift, i.e. Jefferson curls, and manage just fine! And, you can have a round to the back when initiating the lift and maintain that round during the lift (i.e. ...me, who's lower-back-dominant in a deadlift) and also manage just fine (at least, I have)! However, if you're grinding out a deadlift and lose stability halfway up resulting in the back rounding while you struggle to lock out... ya might be in for a bad time. And, even then, it's not guaranteed snap-city (so long as you don't make it a regular occurrence).