[atoav]

Nope, you don't add /s because you think the sarcasm is generally not recognizable otherwise, you add it because of accessability and to avoid spawning useless debates.

Not all people in all situations will read a thing as sarcastic — e.g. your response to a post might be sarcastic, but the three comments next to it which were not there when you wrote it, might produce a context in which it would be read as serious by most people without neither you or your readers being at fault. But you have the power to clarify and at least on this platform a unneeded /s has more benefits than disadvantages IMO.