BOAC
Well, here's a story that might illustrate why you would "maintain last assigned altitude until on a published segment". Inbound to NAS Rota, Spain (LERT) in the vicinity of the Martin VOR and in cloud, I was attending to physiological needs and didn't hear the clearance, but when I returned to the flight deck of the C-5, we were descending. It seemed a bit early to me, so I asked what altitude we were descending to and why so early. The two other pilots said Sevilla cleared us direct to Rota, descend to 4,000, report leaving FL70. I didn't like going down so early and told them to climb back to FL80, our last assigned level. Just as the words left my mouth, the GPWS sounded, "Terrain, Terrain". We climbed, I informed ATC that we would maintain FL80 until Rota. I also asked what the minimum instrument altitude was for the area and never got much of an answer, let alone an actual altitude.
Lesson: never, ever descend when off-route without being absolutely sure of the terrain. And then don't!
GF