For drop shadows:
This depends on where the main light source was when you took the image. Choose the shadow placement on the opposite side of your light source. For example, choose the bottom left shadow placement if your lighting is in the top right corner.
You can also check where the shadows fall on your images and choose the same placement in your template.
For reflections:
The most common placement is Bottom - but depending on the angle at which your product is shot, you may also choose Bottom-right or Bottom-left.
