Some forms of competition require a certain grip and stance, so if you are intent on bullseye competition, look at the rules.
"Practical" application of the handgun, whether for self-defense or competition, require multiple grips and stances. At a single IDPA or USPSA match, or at a shooting school class, you might shoot freestyle, strong-hand only, weak-hand only, kneeling, prone, etc., so you really need to be able to do all.
As for the grip, the consensus is that you want the gun nestled as deeply into your hand or hands as possible, to help counter recoil.
Current thinking is that when holding the gun with two hands, you can get a better overall grip by making more room available for the weak hand by keeping the strong-hand thumb high on the frame.