In most cases, it doesn't matter. Once you climb onto your bike and start to ride, that front wheel has to handle forces way greater than the weight of your bike just hanging statically.

If you have a mountain bike that uses an oil-filled suspension, a FEW of those don't take kindly to hanging at odd angles - they'll leak.

If you are picky about esthetics, keep an eye on the plastic coating on the hook. It can wear away and allow the metal hook to leave cosmetic marks on your rim.