Can you suggest some resources that provide guidelines for choosing tags/attributes?

Hm. I've never used any. The issues I ran into were along the lines of SOAP output with XML structures that alternated within the same document using attributes or child nodes for properties of the object (likely, the overall response was a result of two different programmers' output and they didn't compare notes).

There were also some instances of things like a parent having a child node and the child having a name sub-node instead of a name attribute on the child, where the parent had a name attribute. I realize there are no inviolable rules about when to use an attribute versus a tag, but what I ran into was inconsistency within a single response object. My point above was that the envelope is a standard, but the contents can be poorly designed and is domain-specific--the same is also true for REST.

My understanding of the standards around SOAP, the ws-* ones, is that the don't promise standardization of the payload. The aim is to provide standards for integration, and those are useful, because tooling and infrastructure around the services can be reused. SOAP standards are there to simplify integration, they say nothing about the payload (other then maybe it having to be xml)