For the absolute best results in preserving the longevity of your bars its best not to store them in your shower between uses.
Due to the abundance of natural glycerin derived from cold process soap they naturally attract moisture. Allowing them to sit in a very humid environment, such as in the shower, may cause them to become pre-maturely soft.
Your handcrafted bar of soap should be allowed to dry between uses. Both the top and underside of the bar should get circulation and be allowed to "breathe". This can be done simply by using a vented or slotted soap dish.