But bear in mind that background noise will affect the level that is read - to counter this, we probably want to measure at a level that wouldn't be pleasant to live with.

Since the perceived frequency response of a given set of amps/speakers tends to change with volume, ideally, you'd want to measure at a level that corresponds to your desired listening volume. But since we don't listen to pure sine waves for entertainment, it's possible that the optimum measurement volume would be different than the actual listening volume. Higher or lower, I don't know...