Both Microsoft Office Mobile and Documents To Go read/save the the DOC/DOCX/XLS/XLSX file formats natively, so there is no conversion necessary. In addition, any formatting/functionality that is not supported by the desktop version, but not supported by the mobile version, is automatically retained. Thus, even though some things may not be visible in the mobile version, you don't lose anything in the file(s).
Feature-wise, lists, images, standard formatting, standard Excel functions, etc are all supported. More advanced features, such as Word Art, Excel's enhanced graphs, etc. are usually the ones that cannot be viewed/edited/created on the mobile version.
Do you think that would suffice for the user's needs?