Technics 1200G VTA Lock or unlock is the question?

Given the precision of manufacture of the device it could be more of a resonance issue with the tonearm(?)
Try strong (sustained) female vocal notes and see how the arm tracks/behaves.

Actually, upon reading back through the responses, I thought LewM was going to conclude by saying that AZIMUTH may be affected (which is a distinct possibility) but rather he refers to VTA.

VTA, I suspect, is unlikely to be noticeable because I doubt the locking mech could inflict a change of sufficient magnitude.
Now azimuth on the other hand...different story...

it's a typical audiophilia 

Audiophilia Nervosa is the term.
Yes the issue might affect both VTA and azimuth.

Okay,  I had a moment.