Tubes are expensive power. With efficiencies less than 89 db or so, you start to need a lot of power to really make the speaker play, although there is a lot of variance due to rooms and taste. So the power rule of thumb thing is generally OK but should not be regarded as cast in concrete.
As far as impedance swings go, that all has to do with the type of speaker and the intention of the designer. For example Sound Labs and older Quads have wide impedance swings but tubes do quite well with them.
With regards to intention of the designer see:
http://www.atma-sphere.com/papers/paradigm_paper2.htmlAs for general guideline/rule of thumb things I can add another one:
"If you are investing in a tube amplifier, your tube amplifier dollar will be best served by a speaker that is at least 8 ohms or more, all other things being equal."
Of course 'all other things' are *never* equal, what this comment is speaking to is that all tube amplifiers whether they have a 4 ohm tap or not will have more power (although not by all that much) lower distortion and possibly wider bandwidth when driving higher impedances.
Bottom line is YMMV, so audition is always the best policy!