Heretics allow you to take a low initial Dom score. If you get past the initial rush phase, place heretics in your border castles. No dominions, absolutely none, will get to a province where you have a dozen or fifty heretics working as lab-slaves/researchers. Mictlan may happily try to blood-sac your dominion to death, and you will just laugh behind your wall of heretics who don't believe in anything
Just make sure you have no heretics in your capital and a few provinces around it. Heretics drop the dom just in the province where they reside.