If it is a standardized administration (according to the rules), there will be absolutely no difference in testing process between the regular norms and extended norms. It's just a question of which tables are used to convert the raw scores.
I would agree that asking about extended norms should only enter the picture after the results are obtained. They are also relevant primarily when there 2+ 19s among the subtest scaled scores, and when the index score in question is near the top of the regular norms (140s and up). If there are no 19s, the extended norms won't change anything. If the index scores are all below 140, then differences resulting from use of the extended norms probably reflect outliers among the subtest scores, which means additional clinical interpretation is necessary anyway.
