Ok so it's not that big a deal, but coaches prefer straight sticks for teaching the fundamentals of puck control, passing and shooting. Not only is a straight stick great for teach proper technique, it allows the player to experiment with which way they want to shoot - Left or Right.
There are three primary arguments for how to determine which way a player should shoot. All are important considerations but ultimately it will come down to player preference and comfort.
Dexterity argument - Players who have a natural dominant hand should use that hand on the top of the stick. The top hand is the control hand for puck control and manual dexterity is important for puck control. This means that a right handed player would shoot left. This can be important for puck control.
Dominant Eye Argument - Most people have a dominant eye. That is, an eye that naturally picks up and tracks moving object better than the other. The difference is subtle, but important. Batters who naturally bat right often have a Dominant Left eye. Right handers catch with their left hand often times because their left eye is dominant and their hand-eye coordination is better to thier left side. This is especially important in Passing and Shooting.
Strength Argument - Many players exhibit a dominant side for speed and strength which may be independant of dexterity. This often occurs natually as a defense to protect their dexterity dominant side. Many right handers for example would prefer to use their left side as a "shield" if you will. They commonly give and receive checks prefernetially with their left shoulder and hip, they commonly skate with a preference for their left skate, stopping for example.
So how do you determine which is the best for your player? All of these arguments are valid and important considerations for a well rounded player. One should not take precedence over another if it makes the player uncomfortable. Learning the fundamentals is tough enough for a young player, they shouldn't be distracted by awkward body mechanics imposed upon them by a coach or parent.
Did we mention that a straight stick is a really excellent teaching tool!?