Mike,
Specifying the name of the parameter in the Creo field in the Attribute data as you pointed out will set up explicit mapping. As Creo parameters are in upper-case then the parameter name specified in that field should also be upper case. Implicit-mapping will occur if an attribute has been associated with the CAD Document type that has the same name and case as the Creo parameter (i.e. Windchill IBA Internal name is TEST and Creo parameter name is TEST).
There are a couple of help topics in the Windchill Help Center that cover the different kinds of mapping. Info can also be found in the Using Creo Parametric with Windchill Guide.
Tom,
Specifying them at the EPM Document Master level means that the attribute values will not be file-based (i.e. all iterations of a particular CAD Document version will have the same attribute value). Additionally, only file-based attributes (those specified on EPM Document/CAD Document) will map to Creo parameters.