Since you are adding a client-side layer between client program and web service, you can design it so that a web service call is never made with "incorrect" arguments. You shouldn't change the value in the wrapper layer, but return an error message to the client program.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com
subject: What should be the behavior of a good wrapper library?