It depends on enemy for me. I always go for at least 1 CSM in the first place. Because 3 Heretic is risky and not durable. CSM gives you pure dps. You should go for CSM first. Now, for enemys,
Imperial Guard : Deserves 2 Heretics (including first one) so you can use grenade launchers against them. Melee Heretics can be focus fired so easly, they don't have a single chance after a few fights. Also, your CSM in the first place.
Eldar : Never go 2 CSM. It rarely works just because of Eldar players missplays. Same menu with IG.
Orks : 2 CSM and 2 Heretics are both cool against Orks. But if you go 2 CSM, you shall never loose your starting tic squad. For 2 tic build, you should have at least 1 anti-melee thing and that can be your Aspiring Champion tic squad. If you have Plague Champion, you can go for 2 grenade launchers in case of crazy dakka fight.
Space Marines : 2 CSM is good. But a big amount of Chaos players prefers to not going for a 3rd squad. They generally build genes quickly and tech fast for Havocs, Noise Marines or Raptors. 2 tic is risky against SM and needs high amounts of experince to make it work. Btw, SM can easly deal with 2 tics.

GK : 2 CSM or 2 tic are both cool vs GK. But GK can counter both of them very well. So you choices can pay off with your T1,5 units like Havocs and stuff. Havocs are a no brainer unit vs GK, as long as you can tie up Storm Troopers from their grenade launchers.
Tyranids : Same build for Orks.
Chaos : It totally depends on your experiance. The problem about mirror fights that both armys are same. So you should make it different as a solution. Go for early pressure wargears or tech fast and get better units.