I often have customers requesting a lighter shaft be installed in their clubs thinking it will give them more distance. The thought is the lighter the club the faster the clubhead speed, therefore more distance. In theory this sounds logical, but it's not always the case.
Each player's swing is different including their tempo. Tempo is the pace of a player's transition from backswing to forward swing. (i.e. Fred Couples has a slow tempo - Nick Price has a fast tempo)
The majority of amateurs that I fit have a faster rather than slower tempo. When these players change to a lighter shaft it often affects their timing and release point which can cause and toe and heel shots. An extra 3 to 5 MPH due to a lighter shaft is of no benefit if the player is not able to hit the ball consistently in the center of the clubface. In most cases this will lead to a loss of distance.
The best way to determine the right shaft is to schedule a fitting where the fitter will have you try a number of shafts that vary in flex and weight. By using impact tape on the face he will be able to determine which shaft results in the most consistent center face contact, as well as launch angle and ball speed.
There is also a subjective factor when it comes to fitting shafts. I will often give a player a club with shafts that vary in weight. After a few minutes they will tell me which shaft feels the best. This too is an important piece of the puzzle in correct shaft fitting.