Azimuth: how critical is it?

lewm

Playing a mono LP with one channel out of phase will tell you more about channel balance, as it pertains to the electronics downstream from the cartridge, than about azimuth, would it not?

No, not at all. You can easily measure the channel balance of each component downstream from the phono cartridge, then factor that into the result of measuring the cartridge channel balance through the same electronics.

 How do you define "correct azimuth", with respect to the placement of the stylus in the groove of a stereo LP ...

In this instance, azimuth is correct when the stylus is located with perfect symmetry in the groove. As others have noted, there can be slight misalignment of the cartridge coils, so aligning the stylus alone may not produce optimum result.

Seems like with a mono LP ... The exact same signal is then fed to two channels in a stereo system. Then when you reverse the phase of one channel, you would still have signal out of that speaker ... To some degree the two would acoustically cancel, but not nearly perfectly, because phase is affected by room reflections ...

You're correct, of course - that's why you don't use speakers when using a mono LP and out-of-phase channel to check azimuth. Use a meter instead.

EVERYTHING makes a difference the better the system it's usually easier to hear these differences. 

The more advanced the stylus shape is, the deeper it goes into the grooves, the more important Azimuth becomes.

Not just for sound, but to avoid groove damage, AND get the advantages of greater surface contact area, longer stylus life.

I start with a mirror the same thickness as an LP. Carefully drop the stylus onto the mirror.

View from the front, the mirror will oppositely reflect, thus reveal any deviation from straight. Now what? I had to put tiny shims on one side for an arm with no adjustment, a total pain in the arse, and you want the headshell tight to the headshell, so tedious, but worth it. 

Also a transparent block with grid of lines is helpful

https://www.amazon.com/gp/product/B07794JXYZ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

this transparent protractor for side view for VTA, Stylus Rake Angle is helpful

https://www.amazon.com/gp/product/B01HF9PEA4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Dover, I could take issue with some elements of your response to my post, Like for example that when you use a meter, you are measuring the sum of any difference in channel balance plus voltage differences due to crosstalk, but it is not important. When I asked how do you measure azimuth, that question was directed to the OP. I was interested to learn why he felt his cartridge was running with incorrect azimuth. I know how I measure azimuth, and I know that you know how you measure azimuth. I was addressing the OP.The point is, as your response also indicates, there are two different criteria for “correct azimuth”. One is the physical model that you describe, and the other is according to the electrical model, where you measure crosstalk in db or voltage . Those two methods can give very different results. For reasons that I and you and some others have already suggested.