With as much as a new paint job costs I'd replace it just to feel safe. Also it is always nice to know what is under the bondo. If it is just a small dent then you've lost a little bit of time... but if there is rust or a badly damaged panel then you've saved yourself a lot of money down the road.
It really is an answer of "it depends." Can you see the backside of the panel? How thick is the filler? WHat kind of shape is the rest of the body in? Personally I would remove it, all kinds of surprises can be hidden with filler. It is not very hard to redo.
I found either a cupped wire brush in a grinder or one of those paint removal pad in a drill work best for removal.
I would and did take all the filler off my 66. It's the not knowing the PO's work that would have bugged me! We've all seen what kind of cheesey fixes that have been done to these cars over the years. Besides, it will give you a chance to work out the small dings.
Depends how important it is to you? And if your painter even willing to go over bodywork that isn't his?
For a daily driver, I'd think if the old paint's been there for a while and didn't reveal problems, the works probably good. Why do it again? No one will ever know but you. I would go to bare metal in areas prone to rust though.
It's a personal decision on how much time, effort and/or money you are willing to put into it. Do you want to do things quick and easy and spend more time driving your car than working on it?