Is it OK to use an 8 Ohm tubed amp to power 16 Ohm speakers?

It is always better to have a higher load resistance - less current is drawn from tube circuits. Compared to transistor amps, tube amps are challenged by low impedances and have difficultly in providing current.

It will be fine but technically it's better to match as close as possible. 

Air Tight is right, trust them. You could try the16 ohm tap on your amplifier and compare the sound to the 8 ohm. It’s possible you may prefer one over the other. That’s a sonic/sound character choice. However either speaker tap is certainly safe and acceptable to use.
Charles

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.