I sometimes autocalc the 1 man things just because it is such a long waste of time on big fortifications. I never use autocalc otherwise. Unfortunately, autocalc often let you off to lightly on losses for massive fortifications. With a Citadel or Fortress armed with cats or cannon you could easily burn off 400 decent troops getting to the king hiding in a corner. And by the time you got there you were so exhausted you could lose another 100 or so before he croaked.

I wouldn't try just my king and a 100 man force. I've seen a single high star general in an upgraded castle (7 star or so, and he didn't have any "runner vice") win an autocalc against a 3 star and a couple of units (on expert.) It was something like 125 men killed with zero losses to the single man defensive army. That was a harsh lesson... Ouch! Use just as many men as you would if you were going to fight it out.