I have 5 Sunfish at my children's camp and we don't have enough storage room to place them flat either on hull or on deck. Is it okay if I store them on their sides leaning at about a 20 degree angle against each other?
I wouldn't trust the boats just leaning up against each other. You might need to take some precautions to keep them from sliding and falling over. However, as long as the boats are covered they should be fine. I remember a Scout camp who stored their boats in their dining hall during the off season, perhaps you might do something similar? Also, just as a final note, you might want to buy a couple of sheets of closed-cell foam at your local Wal-Mart. Hikers use this stuff to sleep on (you might already have a couple in use as excercise mats), it only costs a couple of bucks, and mine (which I bought at Wal-Mart) is blue. Anyhow, I would cut it in to strips to put under the rub rails, this can help protect them. I would also use the foam to protect any other surface of the boat which contacts another surface, like if you leaned the boat up against a wall.
Our Scout camp stores them that way. A few caveats. Make sure that there is sufficient foam BETWEEN the boats so that the hull of one isn't pressing on the splashguard of the one below it. I've seen boats slip and crack the splashguard of the one below it or get gouged by jam-cleat. If possible I would store them as upright as you can with the drain plug open and down for drying. Take some rope and tie the last boat in line to the wall by using the front handle and the rear bridle. This way they shouldn't move during the winter.