Speaker Impedance, Tenor OTLs, and Watt Puppies

Generally, is it better to match Tenor OTLs, or any low wattage tube amp, with 8 ohm speakers, rather than say, 4 ohm speakers? I do not clearly understand the impedance factor concept, but have been told that 8 ohm speakers are easier to drive, and also that speaker ohm stability is important. Are Watt Puppies a poor match for the Tenors? I know they will sound good, but are the Watt Puppies performance being maximized?