If you run a 16 ohm speaker on an 8 ohm tap, you may get around a 3 decibel loss in volume. On some amps, this can be a risk. It depends how they wound their transformers, and to what impedance they're loading the tubes to, plus other design characteristics in the amp. I would ask first to be safe. Some amps may be fine this way, others won't be. When the speaker impedance gets too high, it could cause the output transformers to arc inside. The worst is no load at all.
