spec-2.0-draft-05: RuntimeContext: public parameters question

Rich,
what is the intended behavior of public params across requests? The spec 
currently states that the portlets should not store these params as the 
consumer will provide them in each request. Does that mean that the 
portlet can count on getting this param from a specific producer after 
the first time it receives this param?
How can the portlet find out which params the consumer will support? E.g 
if the portlet knows that the consumer will not provide the zip code it 
can output a msg and a link to the edit mode to preset some zip code. 
When it knows that the consumer provides the zip code but the current 
request is lacking it the portlet can output some warning that the zip 
code was not available.

Stefan