I usually only apologize if I thought it was flagrant. Like you come around a corner and whamo, warhammer right in the brain, because I swung at a face and didn't stop to look for a symbol. Or I hit you with a throwing axe because your symbol color kinda blended with the background and I just didn't notice it.
Basically I say sorry if I meant, "That was bad and I should be capable of not repeating that type of mistake in the future".
If it was just a happenstance hit in the heat of battle, while genuinely trying to avoid hitting you, then I don't generally give or expect an apology. It happens.