Yes after the unibody was painted from the front (wheelwell) to the rear valance. Some factories had the rear valance installed at the time of exterior paint application just not pull into the main body Some years and factories you can find semi-gloss blackout overpray on the rear valance.
In general you always back mask this to produce a soft edge its the safest choice. There appears that some (factory and year) may have been brushed on but I've got to find more examples.
How far up the rocker panel? It was originally applied varied Safest thing to do is to leave about 1/2" on the bottom edge of the rocker
Remember that the overspray from this application should cover the floor panel and anything the would hang down that was in place at the time.