Pins are essential part of the stained glass grinding process because if your pieces are free to move all over the pattern you can't assume that they will line up correctly when you think you are finished. Nowadays all of our students agree but I still see people pinning pieces incorrectly. If the pins are placed at an angle the glass can (and will) easily ride up on top of the pins thus defeating their purpose. Below is an example of a piece of glass that is not secured.
Next we have an example of superior pin placement. The pins are straight and will anchor the piece firmly as neighboring pieces are added and removed throughout the grinding process. Also remember to push the pins thoroughly into the work surface. Your pins need to go at least 1/4 of an inch into the work surface to truly hold your glass in place.