I suppose you can store your rattle cans anyway you like. It won't hurt the paint but all the paint inside and the rattle ball will gather to that side of the can. When you start to shake it, it will have to start mixing from the side that it was laying on, instead of the top or bottom. I store mine on their top, so when I turn them over to shake them, they start to mix up and down in the can,not from the side.
It's just my opinion that it will mix faster when the paint is already in the main stream, then you have to get it to feed into the main strea from the side.
Jeff