I remember keeping mental notes of this, but moral increases do happen. Had Goriad in Eidulb fighting battle after battle against rogues. Most times even without casulties (important to note I think, never had casulties) I saw a moral decrease after the battles, but sometimes it would randomly increase after a battle.
Did some quick check with my character on BT who has been fighting monster/undead hordes regularly on his own with Archers. He always takes out the hordes without getting any casualty, but the morale loss usually is around 8%, sometimes only just as much as 1% though.
Although, this is 1 unit. As Delvin pointed out, there can be differences in likings from unit to unit. That's why would need some more statistics. I don't always check, but I haven't seen much morale increasing in my BM life either. At least to my best knowledge.
