I've never heard that there is a correct answer.
What I have heard/read is that most amps have a "sweet spot". Bryston SST2 amps though, are supposed to have the sweet spot all the way through the power band. So, I'd guess you'd want to hit your amps sweet spot (typically 1/3 power??) to test.
The practical side of me though says that the answer to your question depends on how you do YOUR critical listening. I don't have a dedicated room, and now I have my first child (he's 13 months old). I don't usually take a sound meter with me, but I know that I calibrate the pink noise at 75 db, and that seems to be the max level I can live with for sustained volume. Frequently, I use even lower volumes. So for me, I'm really concerned with how my system performs at modest levels. It sounds great louder (I get to dabble on rare occasions when my wife and son are away on errands), but most of my time is lower volume, so that's my focus.
Speaking of which, I currently have a Krell S1500 amp, but I'm hoping to sell it and get a 9b-sst2 soon. I used to be concerned about the 200 watt/ch @ 4 ohm limit on the 9b-sst2... then I did some math and found out there's probably no way I'll ever drive my speakers loud enough to use that, and that without a dedicated circuit, i'll never even be able to pull 200 watts/ch x5 anyway.

But I DO want to obtain that whole power range sweet spot goodness so i can enjoy the best my system has to offer even at low levels.
- Garrett