Jeans can be folded or hung.
However, if you are conscious about preserving the wear pattern of your jeans, it is preferable to hang with a clipped belt hanger by the waistband or, in a hurry, by a belt loop.
Jeans are made from denim which is a very sturdy textile that is good at maintaining its shape. Jeans are designed to be work pants that be subjected to stress and are more forgiving to sub optimal care and storage.
That being said, proper storage and care will result in a longer-lasting, better looking, or better fitting garment. Raw denim enthusiasts, in particular, take pride in the wear patterns developed over time which can be ruined by improper storage.
Hanging with a clipped belt hanger is preferred over folding as it eliminates any creases that would be caused by folding. When hanging, its important to hang from the waistband (as opposed to the hem, or bottom of the pant) as it is a durable and reinforced section of the jean.