For you lot out there who've worked with XML, I've got some questions about convention.

All of my XML handling has been done with PHP. PHP's general approach to XML is fairly casual: so long as it validates as valid XML, it doesn't care what elements there are. I imagine XSD validation is available, but optional.

Currently, I'm making calls to an ASP.NET box. It seems to have this habit of requiring XML that conforms extremely closly to the XSD. In practice this means that tags it doesn't know means the whole message blows up. I'm finding that this is not very flexible. However, my counterparts on the .NET box seem to have trouble understanding that this behaviour could be different.

When building custom XML inter-process handling, how common is it in your experiences for the XML to be required to be thoroughly conform to the XSD?

Wade.