XML newbie question: processing order of XML elements

["Ralf M." <ralfml@alfray.com>]

Hi!
I have an XML newbie question regarding processing order of XML 
elements, I believe also called sequencing vs non-sequencing in XML.

Background: Until recently, I was told multiple times that element order 
should not matter in XML, whereas I always believed it should.
Well actually I always believed it should be meta semantic associated 
with the XML which was not expressed by the DTD nor the Schema (I still 
have a rather limited understanding of those, sorry). Reading the XML 
spec 1.0 again I find out that:
- Element content (http://www.w3.org/TR/REC-xml#sec-element-content) 
nodes contain other elements have an implicity ordering. The DTD 
described the grammar (choices, sequences, mixed).
- Mixed element (http://www.w3.org/TR/REC-xml#sec-mixed-content) nodes 
can contain both character data and child elements. In this case the DTD 
can constraint the type of the child elements but not their order nor 
their number.

I fail to see why the mixed elements could not constraint that, but 
that's not my question here.
I don't feel that the spec clearly precise what the user of the XML 
should do. Is it ok to process child nodes of a mixed element in 
arbitrary order? In real life, I do not think that would be wise (f.ex. 
<font> tags in an HTML <p>...).

So when I design my XML, should I expect all the target applications 
(especially the one I can't control) to process all the non-mixed 
elements in order, or should I provide some kind of mechanism to 
identify the order (I noticed RDF/XML has this rdf:seq in RSS 1.0, which 
looks redundant if element order is expected to be respected).
In my case I need to load some custom XML, process it (actually perform 
a simplistic diff on elements) and write it back. I'd like to know if 
generally speaking the ordering of elements should be retained or if the 
processing can reorder them as is most convenient. I'd like to avoid 
having to parse a DTD or a Schema if any.

Comments, bashing or references appreciated.
R/