gravity + the weight of the boat will cause the weak hull surface to deform over time. plastic tends to be much more rigid over a "curve", the tighter the stronger, & the hull is always the flattest, most bendy part of your boat.
so, if you have a playboat & like the "looseness" of the planing hull (to release on green water), then to preserve the integrity of the planing surface as well as the overall shape the boat designer had in mind, you should stand it up on end, or "cradle" it. same holds true for creekboats with displacement hulls. oilcanning sux.
now that i think about it, 3 of my 4 boats are rotting away on their hulls ...