I can't hear any distortion on my receiver no matter how loud I go. I guess my ears suck.
% alone does not tell the whole story. It depends on the contents of the harmonics. 1% THD for one amp may be harder to detect than 0.5% THD for another. If the THD is below 0.1% from 20-20,000 Hz, chances are that unless you have the so called golden ears, the distortions should not bother you at all. The T758's THD stayed below 0.1% until about 100 W, that's a little high compared to their integrated amps but I am sure it is low enough to be practically irrelevant.
There are other types of distortions such as inter modulation distortions that are more offensive, but as Gene mentioned more than once, if an amp's THD is good, chances are that it's IMD would be good too. To me, all else being equal, lower THD is like output power, only opposite in terms of magnitude. That is, the lower the better regardless. If THD+N is lower than say, 0.0003% (e.g. Benchmark AHB2), you don't have to worry about the contents of the harmonics, odd, even, low or high order, who care if the total is practically zero? That is not to say there is anything wrong going with one that has higher THD, if it has other positive attributes that are more important for better overall sound quality.