Reciprocity is a true property. After all we measure magnitudes ratio and relative phase. I prefer to use roving accelerometer for the following reason: one wants to make sure that he is NOT impacting a node prior to the test. Once you have verified that it is not the case and you ARE in fact inparting energy, you can proceed with the test. Fixed impact location also is convenient to ensure that direction of impact has components in all 3 axes.