LOL... I knew that was coming!
I don't know, but for me, consistency is the key thing. And in my opinion, the only way that you can do it consistent is to smoosh the plastic plate down hard on the glass.
If you get the calibration fluid (53.0 mS conductivity calibration fluid will read 35.0 ppt) and calibrate it with whatever method (gentle lowering or hard smoosh), and repeat that method when you take readings, it really doesn't matter which method is "correct" since you've calibrated the unit using that method.