I've had one cardio day this year and that was in March. I think regular cardio is important so I think I'll make March cardio a regular event from now on.
Just to back up doing cardio after a leg workout Chris Carter of the vikings says he does cardio in his pool and stretches after every game when he played and workout and that helps with the soreness. What I found interesting he does this for 6 hours. He trains a lot of atheletes and has a nice website.
I would suggest as mentioned above do a little cardio and add some stretching exercises since you have broken down some muscle fibers before so it needs to heal and stretching will elongate the muscle while it is repairing itself.
Definitely helps get out the lactic acid. I'd say if you're sore it's fine but if ur half crippled and it hurts just to touch your quad, maybe take it easy. Lol