Answers should be adapted to the experience level of the asker, and they should state clearly the dangers that are associated with a specific experiment or reaction. If I have the impression that the asker does not have the necessary expertise for a certain reaction I'd strongly warn about this and I would emphasize the dangers of the reaction.
But I don't think we have to extensively warn about the dangers of every chemical mentioned in an answer. I would mention any unusally dangerous chemicals, but if you're answering an experienced chemist there is no need to spell out every single possible danger. For an amateur even simple chemical compounds like diethyl ether would need a warning, this would be excessive in an answer to a professional. But for more dangerous stuff like e.g. BuLi I'd probably throw in a warning even if I'm pretty sure that the asker knows about the danger.
I don't think a general warning label is useful, how dangerous a procedure is depends strongly on the experience of the person performing it. The difference between what I would consider dangerous enough to warn an amateur about and what a professional chemist would consider dangerous is pretty large, and no simple warning system can accomodate that difference.
What might be useful would be integration the safety information of all chemical compounds in a post and display the danger signs as well as the GHS hazard statements. But that would be a huge modification, and I don't think this would be possible any time soon.