Req 1.15 Representation of invalid XML characters

Hi all,  I was implementing the <cp> element in our experimental XLIFF 2.0 library and I have a question:  We've discussed the representation of invalid XML characters in the text content so far. But what about the original data?  --a) We could have invalid characters inside a <sc> element for example if the special characters are part of the original data that is abstracted as inline code.  --b) We could have invalid characters inside also in the <data> element used to store outside the content.  Should we force the original data to be coded with <cp> as well? Or should that be the responsibility of the tool? Note that often such content is not used for interoperable operations.  Any thoughts? -yves