It's perfectly fine to store skis standing up, as long as you take a few precautions. First, make sure that the skis are securely attached to something so they don't fall over. Second, if possible, store them in a cool, dry place out of direct sunlight. This will help prevent the skis from drying out and becoming brittle. Finally, if you're storing multiple pairs of skis, be sure to label them so you know which ones are which!