It doesn't break any rules, but it's unfair to the village owner or guild members who have to waste their resources on it!
[[File:Attack_Example.jpg|link=|none]]
As such, it is impolite to attack legions and leave them with low health. There is no official "limit" on how much health should remain. The strength of the legion plus 19 would be a value that can serve as a very good guide.