There are only 2 guarantees in the concrete business -- 1. It will get hard and ----- 2. It will crack. Just as long as there was proper rebar/wire installation installed within the pour and the rebar/wire is within the pour and not just laying on the ground, the hairline crack should be no worry because the steel will hold it together. Some contractors use wire - wire is better than nothing but does not have the tensile strength that regular rebar has. A good concrete prep job consists of No. 3 steel bars installed in a 18" grid. As far as only sand underneath the concrete pour, having 1 3/4" rock is much better but that depends on your individual building code. Again, no matter what you do, concrete will crack. Mother nature always wins. All you can do is try to fight her back as much as possible.