You can check by measuring on-axis and vertical -10deg (or +10deg if the mid woofer is above the tweeter). If the -10deg SPL is around the crossover frequency "above" the on-axis measurement (better phase summation of tweeter and mid woofer), then the loudspeaker developer used an axis between tweeter/mid driver as reference (or ignored the phase shift at all ).And what about 3-way? Also at tweeter axis or between tweeter or mid driver?
The relative time delay (different distance to the ear) between mid woofer and woofer is, other then by tweeter/mid woofer, in most cases unproblematic, because of the low crossover frequency. For a 200-400Hz crossover the phase shift could be ignored (<15deg, for speaker with normal dimensions).
It really matters when the crossover frequency is high and the distance between mid woofer and tweeter is not narrow. For example, B&W speakers with 6-7'' mid woofer and crossover at 3-3.5kHz.
Here a simulation of a 2-Way-Speaker 25cm x 40cm (same for 3-way) with a second order Butterworth crossover at 3.5kHz. Goal is a warm sounding hifi-speaker, so we want a reduced power response around the ear channel resonance (around 2.7kHz). Developed for hearing between tweeter and mid driver:
It's obvious, measuring on tweeter axis (-10deg vert) would not be the best starting point.
We use an even order crossover, so the radiation lobe at the crossover frequency isn't tilted:
But as Juhazi said, reality is far more complicated:
Tall speakers often have the tweeter on top, above listener's ear level. This may help to get best phase match at listening spot height! A tilted or stepped baffle is used to set back the tweeter which helps delay matching. ...
Designers with modern methods must define optimal design axis, but there is no fixed location for that. Common practise is to set it at midline between tweeter and mid for 3-ways. Some 2-ways have it on tweeter axis, others at midpoint between woofer and tweeter like KH80.