The lower the frequency, the more current is needed to drive the speaker to the same volume, so it's generally more important the lower in frequency the impedance is.
Its not uncommon for some manufacturers to state a speaker is "8 ohms" but if you look at an actual impedance graph for that speaker, the woofers average closer to 4 ohms, while the tweeter is somewhere closer to 12-16 ohms. So in reality, such a speaker is a 4 ohm speaker as that's where the most current will be demanded. It's better to have the woofers average 8 ohms and the tweeter average 4 ohms as the tweeter needs much less current. Generally speaking tho, a more balanced impedance load is the ideal.
Though it can also depend on the amp, as some amps will react differently to speakers with miss-matched impedances.
It's rare to see a speaker with an average higher than 8 ohms, some older Altec type speakers averaged around 16 ohm.
So excluding peaks to 50+ ohms from rises from port tuning, FS or a high impedance load down low (like the 2.4 with 4 woofers in series) high impedance generally isn't a problem.
You may notice lower output in those regions, but it really depending on how the amp handles it. (some amps are more reactive to differences in impedance load, while other amps are less reactive)
