You're supposed to use chars for character data, in which case it's not supposed to matter whether or not the integer representation is signed or unsigned. All you generally do with characters is store, compare for equality, and print. Those should all be safe to do without ever knowing if the underlying integer is considered signed or unsigned.

It's only when you start doing arithmetic on characters, or just want to use char to mean "byte" (or "octet") that it matters, and then it's a very good idea to be specific and say "unsigned char" if that is what you expect.