You can use whatever you want! I like squares and circles because it allows me to find whatever image I want and transfer it onto a "neutral" shape.
The KopyKake is also great when you dont have a cookie cutter, but you still want a specific design. I used it to cut all of the carnival cookies in my photos. Just put the whole sheet of rolled cookie dough (I use it straight from the freezer) under the projector and cut your shape
This way, your cookie is always the perfect size to match your design.