Like others have said we don't teach random "preventative" shocking, instead we teach testing and dosing as needed
To sum things up, our method is to test the water regularly (multiple times per week on the major items, probably even daily on FC, CC and pH until you get a feel for your pool, and every week or two on the more stable things like TA and CH), then only add the chemicals that are needed by the pool as a result of testing.
The pool store method tends to be, let them test the water in detail every couple of weeks so they can sell you more stuff to put in the water that you don't really need, then have you do rough imprecise testing for chlorine and pH between visits, keeping Chlorine in a broad range (mostly at levels we consider dangerously low), then once per week blindly boost chlorine level to "shock" level to hopefully make up for anything that may have started growing due to the low and imprecisely track chlorine levels with no testing to confirm that shocking was held high enough or long enough to finish the job.
This is win - win for the pool stores, this process causes you to use more chemicals, often have a less than perfect looking pool at various points on this roller coaster ride, giving them a chance to sell that expensive magic in a bottle (phosphate remover, algaecide, etc.), and hey if it all comes tumbling down when something did start to grow and the weekly "shock" was not long enough to kill it off, they get to sell you another arm load of chemicals to "fix" the problem.
Ike