No, when you create a protocol define exactly what you need what is optional and what is an error, and stick to that.

Being liberal on what you accept is a path for disaster.

You changed the problem. Postel's law is not about writing the protocol but implementing it.

Sure, protocol should be designed to be as specific as possible but unfortunately these are not always defined up to that point for any good or bad reasons, and we generally are at best just in the implementation side and cannot influence the writing of the protocol, so the Postel's law is the best we can apply to avoid having to cooperate with the rest of the planet.