I have installed stair treads both ways and it always cups up. I have given up on following the diagram in Michal's post. Heart up does not wear as well. The key is to seal all sides of the wood, fasten it well and if using 2x stock, kerf the bottom. Cupping is definitely due to weathering on exterior stair treads. The sun dries the top and the bottoms stay damp. The treads on my shady north stair haven't moved, while the south ones I am going to go back and screw'em down and plug them.
