In the 1990s and 2000s as the USPS moved to self-adhesive stamps, did you ever notice that the die cuts often extend all the way through the margins? This made separating a plate block easier.
I noticed that the recent Planets of our Solar System issue and other recent issues have die cuts around the stamps, but there are no die cuts into the margins. With the backing paper and the thick paper used to print the stamps, I find it very difficult to just fold the margin piece and tear. The safest way will be to use scissors and cut through most of the margin, up to the beginning of the die cuts.