I don't think people sieging forts should ever get terrain bonuses regardless of sorties: if you're sieging a fort you're stuck in one place and can't manuever to take advantage of terrain, and no one is going to put a fort in a position where the terrain perfectly shields a besieging army.
Yeah that's my opinion too.
In multiplayer it's quite convenient to take up defensive position in enemy territory while you siege their province, then the enemy won't attack you because you're on mountains or whatever and has no choice but to let the siege going on, or try to maneuveur around your forces or mountains. Which can be almost impossible in, say, persia.
Now that mountains are a 100% thing that's will even more reinforce this behavior.