Polo shirts should be folded and NOT hung.
Polo shirts are commonly made with a knit fabric called pique or occasionally jersey. Knitted fabrics stretch by nature which provides a comfortable fit while wearing. However, this also means that they may stretch in an undesirable manner if stored improperly.
If you hang your Polo shirt, the weight of the fabric may result in your shirt stretching against the shape of your hanger creating a disproportional look around the shoulders.
The general rule of thumb is that clothing made from woven fabrics can (and often should) be hung while clothing made from knit fabrics should be folded.