How often do you check tracking force?

yes, I re-check often,

I double check everything, overhang, null points, azimuth, arm parallel, tracking,

and then I start with anti-skate set using the blank side of my protractor lp, then refine by ear using familiar content with recognizable l/c/r imaging,

1st the cd version to hear what the engineers intended, and make sure my system's balance is good, then the LP, adjust anti-skate, using content on the outer or inner tracks, I just learned that inner skate lessens in the middle of the arc.

I have two answers.  On two turntables, both with fixed head shells, once set up the variables do not change.  I re-verify the settings, but nothing ever changes on either one between stylus examination under microscope.  I do that annually on one and semi-annually on the other based on usage.  The third turntable has a removable head shell and cartridge changes are frequent.  On that one, like Elliott, I check and adjust frequently all parameters.  I use analogMagik to set up and adjust all parameters and examine all styluii using a Wild-Herrbrugg microscope either an M3 or an M5.

I recheck once in a while. Didn’t discover any deviation. 

Absolutely, with the VPI unipovot