I have to disagree.
Matching impedance is always the safest, most reliable way to run a tube amp, but if you MUST mismatch it's always safer to mismatch low, not high. Like I said, current through a tube power amp is limited by the tubes themselves, therefore lowering the impedance won't cause them to draw more current than they're capable of handling. It will increase wear on the tubes, but it won't destroy them - at least not suddenly.
On the other hand, if you raise the impedance of the speaker too high, you run the risk of damaging the OT. This is because electricity always tries to find the easiest path to ground. If the impedance of the speaker is too high, the electrons will start taking the easier route of bridging the transformer, which turns it into an inductor. When this inductor discharges, bam - fried OT, at minimum.
The worst possible thing you can do to a tube amp is run it with no speaker hooked up (i.e. infinite impedance). This is the quickest way to ruin your OT and possibly other components. Short circuiting your amp (i.e. zero impedance) is far easier to deal with than this scenario (but still not a good idea, because it will ruin your tubes pretty quickly).
Bottom line: mismatching low will wear out your tubes a lot faster, but mismatching high will destroy your OT. Tubes are far easier and cheaper to replace than a transformer.
I think that a lot of misconceptions about this topic come from the fact that tube and solid state amps have pretty much opposite behaviour when the speaker load changes. Transistors aren't self-limiting like tubes are, so if the impedance is too low they will fry.
