It depends on the amp design and the standard of component. It was suggested in the manual for my old 100W Mesa Nomad, that in the 50W/half power mode, you plugged the 8ohm speaker into the 4 ohm output for the correct impedance match (with only two valves working). But it also said that it was fine to leave it in the 8 ohm jack and that it wouldn't cause any damage, you'd just get a fatter, darker sound. But note that it was an output transformer rated for at least 100W, which would be running at 55W, so the mismatch wasn't going to cause the transformer any issues. (at least at one speaker ohmage step difference).