I mostly agree. In many, perhaps most cases I'd rather not specify that the input to a function is UTF-8; I'd prefer to say it's an array of bytes that will usually be valid UTF-8.

Even valid UTF-8 can turn out to be fairly unusable because it requires unimplemented features such as some corner case of bidirectional text, unexpected combinations of diacritics, or just plain characters that were added to the Unicode standard after your font was designed, so leaving validation till the point of rendering can be a good strategy, and it also means that your validation will be even more ridiculously fast in the case where the string never gets rendered at all.