Seems like a classic time to apply the "internet philosophy" of being
conservative in what you send and liberal in what you accept. I'll
bet dollars to doughnuts that there are plenty of implementations out
there that go down in flames when they see NULLs. Yours, of course,
should not do that, but neither should you transmit NULLs if you can
avoid it. For my part, I tend towards extreme paranoia, and would
happily recommend that if your message contains any characters <= ESC or
= 128, you should use a content-transfer-encoding.
In other words, my vote is to always accept the NULLs but never generate
them. Whether you want to "fix" them in things you accept is another
story....