The "special" units gained from tournaments such as Praetorian Guards and Skyclad Warriors show up at the top of the list when assigning units to armies. I would imagine that these units are probably among the last units most players would want to assign to armies. They "get in the way" all the time for me, and I'm constantly afraid I'm going to accidentally annihilate my skyclad by asking them to actually fight.
I think it would be nice if these "special" units could be reassigned to lower ranks in the Assign Units list in order to keep them from inadvertent harm.
Not a super high priority, I'm sure, but it would be nice to have, especially if there is a really easy way to do it (like a code for units with upkeep = 0).