I've thrown away a lot of really nice ported OE heads that made pretty good power and the owners were pretty happy with how the cars ran. Some of them I ported, some were ported at other shops.
For big blocks, when they're heavily modified(especially with big intake valves), the two most common places for cracking is across the intake seats into the bowls, and externally under the two center exhaust ports.

Of course, if you're racing in a class where they're required, you don't have a lot of options;
-Port them less(go slower)
-Don't port them at all(go even slower)
-Accept that the heads may not last all that long.