- Don't.
(Update: link display problem fixed.)
Counterexamples (promoted/high-karma/high-status author):
(I began with the first one and ended with the last one for a reason. I am entirely in favor of applause lights and positive-affect words as long as they serve their purpose -- community-building, making humans feel good -- and don't substitute for argument where the latter is appropriate.)
OK, a more accurate guide would be “Don't, unless you're discussing rationality itself (as opposed to an application thereof) or in other unusual circumstances.” But still.