There are many tricks - some of them, like tasting plaster are on a border of being superstitious. I taste paint for lime. Human sences are the mysteriose thing, tast is perhaps one of the most trasted, so you have to tast a thing to be sure that it is right - even if it does not make any sence. Brush strocks in fresco are en essential part, in fact only fresco gives such thin strokes that of a single hair. And I guess the brushstrokes indicate why crosshatching is mentioned in many instructions.
Plaster is very delicate and sensitive to the amount of water it receives through the brush.
Another possible purpose to tasting the plaster is that sugar may have been added to the lime. For some reason sugar causes the lime to accept up 14 times more water - at the price of substantially weakening it at this high level. I hope you do not mind, but I had to clarify this, about the gypsum. It might be confusing. Gypsum plasters are used for ornamentation elements and such.