When I am doing cardio regularly, I don't do it on leg day. (I can't...*LOL*) But the days before and after, I do. I like to do it on the days after leg day. I've found it helps my legs recover a little quicker. Sure it hurts, but the payoff is worth it to me. (Being not as sore for as many days.)
For what it's worth....
Joker