How much pressure is TOO much when handling playing cards?
When you start out learning card magic, one of the cardinal sins is 'gripping' the cards too tightly. You often get so caught up in practicing the steps of a routine (be it sleights or just the basic dealing/selection/shuffling processes) that you tense up and squeeze the deck like a sea eagle trying not to drop a wiggling salmon. This is bad form. Tension looks suspicious. Plus it inhibits good technique.
The very best card magicians have a gentle 'flow' about the way they handle a deck. Everything is light. Relaxed. If you want to be a great handler of cards I would humbly suggest that you make this a priority to master. The mantra I always repeat (as taught to me by the peerless Ben Earl) is - 'Hold the cards as lightly as possible... and then a LITTLE LIGHTER!'). Just keep that going round your brain as you practice your tricks and techniques. Eventually you will find that you develop a lightness of touch that permeates ALL your card handling.
And remember - the more relaxed you are, the more relaxed your audience will be. If your whole demeanor is light and airy, your spectators will develop an inherent trust that you are in control of the situation, and that will dissipate any constriction, suspicion or undue focus on your handling of the cards.
And, if you are in any doubt as to how much pressure is TOO much on a deck, check this out...