T-shirts should (generally) be folded and NOT hung.
T-shirts are generally made with a knit fabric called 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 T-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.
If your t-shirt is made from a woven fabric (rare) they can, and should, be hung.