The proper way to store your plastic kayak is vertically, on its stern. If you store it horizontal on a flat surface, the hull will potentially oil can. You could also stress the hull by suspending it on the ends. This is especially true if your garage gets hot in the summer. If you have a creek boat, you are going to trash it anyway, so it won't matter how you store it. I thought Jackson kayaks had to have their own bed and down comforter. That and some super cool music or they start to rust.
Do unto those downstream as you would have those upstream do unto you.