The spokes are ALWAYS resisting forces pulling in that direction. It's the spokes on the TOP half of the wheel that take the weight in normal riding. With the amount of tension in the wheel (I don't know how much, but I bet it's a ton per wheel easily), the weight of the bike is inconsequential. I bet you could hang YOUR weight from it for a year and not see any deformation. If you start pushing sideways on the bike while it's on a hook, yeah, that could cause damage, but not straight pulls.