By and large, follow Postel’s law on this (http://en.wikipedia.org/wiki/Robustness_principle). As others have also said, though, make it not-too-hard to do things that your type system would otherwise restrict, because if I have to interact with a misbehaving server (including to the point where I have to send what would otherwise be invalid header data), then your strictness would make it too hard to do so.