There is no chance of physical harm to either the amp or the speaker to use any tap on a tube amplifier, so it is worthwhile experimenting to see which sounds the best. One combination may sound good under one set of circumstances, but not another (e.g., one one combination might run out of gas earlier than another so it would not be as good when required to play loud, but, it may otherwise sound better).
If you actually do have an unusually high impedance speaker, an amp with an unusual 16 ohm tap might offer a theoretical advantage. I presume that 16 ohm tap means a smaller turn ratio in the output transformer which may mean better sound. Again, it is a "may" because you might prefer the slightly better bass control and smoother frequency balance of using the 8 ohm tap instead (higher damping factor). Again, when you have the time to make the internal switch, just try it.
If you actually do have an unusually high impedance speaker, an amp with an unusual 16 ohm tap might offer a theoretical advantage. I presume that 16 ohm tap means a smaller turn ratio in the output transformer which may mean better sound. Again, it is a "may" because you might prefer the slightly better bass control and smoother frequency balance of using the 8 ohm tap instead (higher damping factor). Again, when you have the time to make the internal switch, just try it.