The key is to replace to the point that you know that it's good metal that you're welding to.
Nothing sucks more than rust popping through from the inside of the panel.
These are pics of a 72 340 Cuda that I'm working on for a buddy.
I had questioned the roof because it had a lot of rust scale on the underside, but it only had some surface light surface rust on the top in some spots.
That was until I stripped out the weatherstrip caulking in the drip rail channels.
It's now getting a new AMD roof skin.