Grip pressure ends up being a personal preference, and I do recommend experimenting with yours. A very loose grip can overcome several different types of flaws in a player's stroke, especially elbow placement oriented. Also, if a player has trouble with long, draw shots or moving the cueball around effortlessly they may want to try holding the cue more firmly. Bustemante holds the cue lightly while doing his practice strokes, but he told me in Monroe Louisiana that tightens at impact (not with pure finesse type shots). If anyone has ever tried to bunt a baseball on a cold day with a loose grip they know what happens. In pool this doesn't happen that dramatically, although transferring energy from your cue to the cue ball is still an important factor.
I like to experiment playing with a tighter grip for many reasons that I can get to later, there's too many players stuck on believing they need to hold the cue lightly, my suggestion is to not be closed minded and don't be afraid to experiment so you know what works best for sure.