Hi Steve,
With a Miles library star you are comparing your spectrum to an actual spectrum of that star so your response correction should be accurate. If you just pick an AxV or BxV type star, take its spectral type from a catalogue and compare your spectrum with a Pickles library spectrum for that type of star, you are assuming that the catalogue type is correct and that star is a normal star of that type. This is not always the case – catalogues can be wrong and some stars are peculiar.
So using a Miles star at the same altitude and as close in azimuth as possible to the target is the best approach. This is not always possible as there is not always a convenient Miles star so sometime you just have to use an A or B type star but remember your response calibration might not be as good in that case.
David