The use of different shapes is essential in pastry. I really love using silicone molds. They offer both versatility and consistency. As you may already know from looking at my pictures...I really like spheres and dots. To get perfect spheres or hemispheres (half circles) you really should use silicone molds. When purchasing silicone molds there are a few things you must consider...what am I going to be molding? Is the mixture hot? Am I going to need to bake or freeze the item? How am I going to unmold the product? These are all questions you must ask yourself before deciding which type of mold to use. There are a few different places where I go to purchase my molds. JB prince in NYC has quite a large selection of nice molds. Another really great source is the Chicago Mold Making School. Here is a link to there geometric mold section of there online store.
Here is a picture of my favorite mold...The popup sphere mold...a single piece sphere no lines...really nice mold.