It's about bolt stretch, not the head material. If you don't torque them as required to stretch the bolt for proper holding power, then the fastener can loosen or otherwise fail to perform. If the underlying part can't take the torque, that's a different problem. the inserts in aluminum heads should not have any problems taking the load.
1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.
