Thanks Robert and sekazi for your comments.
So I'll make sure I get a case with magnets. Glad to hear that it is a usual feature of all cases, even unofficial ones.
How big is the risk of the case opening up when it's jumbling around my bag/backpack. Is the magnet strong enough to keep the case from flapping open? I don't want to get a scratch on the screen.
Are some cases' magnets stronger than others? Are some cases more secure (less prone to accidentally opening) than others?