I have follwed the steps you outlined and from what i see the results are the same whether selcting the mapping file or not.
I have attached the STAAD file that was exported also. Not sure if I mentioned this before but under my current installation the export didn;t work at all and I had to manually register the OpenSTAAD dll. This is the path to the DLL
C:\Program Files (x86)\Common Files\Bentley Shared\IEG\OpenSTAADBentley
whereas the path to the mapping file is here:
C:\Documents and Settings\all users\Application Data\bentley\MicroStation V8i SS2 (08.11.07.443)\WorkSpace\TriForma\stf_imperial\data
and the us.xml file is here:
C:\Documents and Settings\all users\Application Data\bentley\MicroStation V8i SS2 (08.11.07.443)\WorkSpace\TriForma\tf_imperial\data
Not sure if that info helps but thought I would mention it.