The answer to this has been covered a few times lately. For a typical 2-way bookshelf speaker, based on crossover limitations, you want to listen at the tweeter level or below. Above the tweeter response can be weird.
So if you turn the speakers to their sides you want to position so that the listener is in the same relative location. Another way to put it, you want your ears to be between the tweeter and mid-woofer axis. Sometimes even on the mid-woofer axis, so arrange the speakers so your location tends to stay in this range.
If you review several Stereophile reviews and take a look at the off-axis plots you’ll see why this is.
