Depending on the design - if you don't mind a slight reduction in power, using the output taps lower than the impedance of your speakers can be beneficial in some cases. Presently I prefer the 4 ohm tap over the 8 in my system where the spkrs impedance ranges from 8-10 ohm. The difference is easily heard. I've heard it go both ways w/several different tube amps w/my speakers.
In the case of my current amp, Roger Modjeski/Music Reference, the designer in his own words, "Although higher idling current will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, and allows for more peak current when needed. The only loss is about 20% of the power rating or 1 dB."
In the case of my current amp, Roger Modjeski/Music Reference, the designer in his own words, "Although higher idling current will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, and allows for more peak current when needed. The only loss is about 20% of the power rating or 1 dB."