My dad would agree with you on that; every time we played Leeds he would spend the whole match muttering about that team, no matter how far in the past it might have been. For example, the famous Worthington Cup game in the 2002/03 Triple Assault Season when he was highly delighted to add the Leeds fans to his charge sheet. Hence at the 2006 promotion celebration game where we were seated on John Street close to the away end: "Look at that 'orrible lot; not only did they have the dirtiest team I've ever seen wi' Bremner and Giles, they'll probably be good enough to start throwing our seats onto t'pitch again."
He would usually go on to explain how he'd always disliked Leeds more than Wednesday. Those who were fortunate enough to experience my dad at a United match can, like me, most likely hear his voice as they read this. By 'eck, I do miss t'owd lad!