They can still crack but far less likely to as Mild Steel is much less susceptible to big variations of expansion during a heat cycle than Stainless Steel. Mind you, most Stainless Steel manifolds are thicker in gauge than the equivalent Mild Steel ones whihc has something to do with it...
And heat wrap...that's fairly wrong, as in most cases, it'll help a manifold crack, not only by trapping moisture and bringing the onset of corrosion, but also by maintaining more heat within the manifold, for greater thermal expansion