Stereophile uses 1%.although I just wish a "line-in-the-sand" %THD figure was used for ASR tests going forward. I use 0.1%, Hypex uses 1%, Amir uses the "knee" which could be essentially anything and varies from product to product.
Now, from the informal testing I have seen, 1% system THD in the treble is about the threshold of audibility with various types of music playing. To account for multiple links in the system chain and a safety net, I personally look at 0.1% THD.