I have some flat skins from wildebeest, bushbuck, springbok, etc. If they are not hanging on the wall what's the best way to store them until needed?

Should they be rolled up to keep from creasing?

Definitely roll up loosely.
Apply a softening oil to the leather side helps too if they are to be stored for more than a few months.

I have some skins like zebra and Bison that I rolled 5 years ago in a closet in the house where the temp is consistant, they look fine.

From a taxidermy standpoint, this is a tough one. Rolling will work but much depends on the type and completeness of the hide being tanned. Many tanneries use alum and over time with varying humidity the alum and degrade into acid and cause the hide to crack and split. Chrome tans are much more forgiving long term, but sometime tend to form a "memory" at won't stay unrolled. If you significant other ever owned a mink coat or stole back in that era, they know the very best way to keep a hide is to have it frozen. If you have a chest freezer where you can lay them out flat, they'll virtually last forever with most tans. Very few professional taxidermy tanneries recommend that hides be "saved" for over a year.