I went through that too....applying materials so "usemtl" statements would show up in the .obj file. Lately, tho, I've been exporting from Carrara undressed, but keeping all groups. In a single vertex object I name vertices, which also end up exporting as seperate groups. The next step is text editor, where I convert most of the "g" statements into "usemtl" statements using find-and-replace. This may sound roundabout but it gives me the control I like.