The question is really not that simple. I guess its a matter of degree for me. Do you mean a dent or a ding? Certainly a nick would be okay to ride as well as a ding but a dent? A dent can mean anything from a dimple to a crater. What are we really talking here, a dint, a furrow, hollow, an impression or is it just plain bent? Really, a picture would be best to judge by.
Many of my rides are dinged, nicked, dimpled, scored and scratched but they still ride nicely. Some old bikes should be perfect, others should show their age, it is part of the bikes history! Original paint with a little rust, a couple of scrapes and a ding or two is just fine with me! Enjoy the ride, forget the value!