Fair hits the same all the way through the move. At the guy above me said, you can reverse it or get a phantom hit.
In order to get a reverse fair, you have to hit them at the beginning, above, and slightly behind Sheik. Reverse hits happen when a move connects with the opponent on the other side of what is considered the "center" of your character. In Melee, horizontal knockback is applied in relation to which way the attacking character is facing/which side of the character you're being hit from.
As far as phantom hits, if a hitbox touches the hurtbox of a character, but doesn't overlap it, the opponent will take half the normal damage and no knockback.