What is important is that the grip pressure remain constant. The best grip pressure for each golfer, is the one that will allow the setup shape (shaft in line with the front arm with a square club face), to return the same. If that takes a firm feeling pressure then do that. My experience has been the lighter pressures allow the wrists (hence club face) to move around and makes it more difficult to return square or even worse, to flip the club head creating off center hits and inconsistent direction.