Why do you store wine bottles on their side or upside down?
Thereof, is it OK to store wine bottles upside down?
Storing your wines horizontally is best. When a bottle is sideways, the wine stays in contact with the cork, keeping it wet so that that cork will not dry out, shrink up and let air get into the wine, causing premature oxidation. Upside down is definitely better than right side up to keep the cork moist.
Accordingly, why do you store wine bottles on their side?
Any time a wine is topped with a cork, it should be stored on its side. A cork is used as a topper because it expands in the neck of the bottle to protect a wine from oxygen. If you store a wine on its side, you can make certain that that cork remains moist (and expanded) at least on that side.
Bottles with screw-top closures or plastic corks, as well as sparkling wines (which keep corks moist due to pressure inside the bottle) can be stored vertically, though horizontal storage is often more compact and practical. Wine storage racks are easy to come by at most home décor shops or wine stores.