Well, I am not much of a body guy..... I leave that to the professionals. But here is what I can tell you. Especially with the front and rear bumbers, you do need to strip them down to the plastic. Reason being is that in most instances, you have cracks in the paint from bumping the front or rear bumper (I guess hence the name"bumper")..... anyway, if you just sand the paint down or scuff it, you might think you got rid of all of those little cracks, nut you didn't. They will show back up again. Maybe not right away, but I bet within 60 to 90 days they will return. So, with the front and rear bumbers, you do need to strip them all the way down in order to prevent the "spider-webbing" that is so popular with these cars. The wheel spoilers are the same way, but worse. If you have cracks in the wheel spoilers, I say just replace them. Those stupid cracks always (most of the time) come back to bite you. Again, not right away, but the materials used to repair the cracks in the spoilers has a habit of wearing out before the paint does and you will see the paint start to either crack or "sink" into the spoiler. The rear spoiler isn't as bad as the wheel spoilers except for the end caps on 1979 to 1981. From 1978 and earlier, they are OK just like the fender air extractors. The rear view mirros are metal so you treat those just like the rest of the car.
With the window trim and t-tops, again, they are metal, so treat them just like the rest of the body. Watch out for the window trim because it is thin, but you can also use a chemical stripper on them rather than sanding them. You might want to do the same for the t-top frames because if you take a sander to them and then all of a sudden slip and sand the glass...... not good.