There are some "guidelines" for Xml Schema: W3C XML Schema Design Patterns: Avoiding Complexity , but not for plain XML I believe.

If your talking about .NET .config files, it's a subject in itself. The implicit guideline is camel casing everywhere, matching .NET classes full type name. The fact that the XML is indeed backed by an XML schema is quite a strong guideline for various reasons: validation, and also intellisense (auto completion) support by XML Tools. For Visual Studio, see this here on SO: How do I get intellisense in app.config for a custom section?