What if I use a detangling rinse-out conditioner with a lot of cones or a leave-in conditioner with a lot of cones? Is heat protectant still necessary?
I'd say YES! Better to be safe than sorry. You wouldn't want to work so hard protecting your natural hair only to have it altered in a matter of minutes.
I agree with NappyNelle; skip the leave-in and use a heat protectant.
John Frieda's regular serum is full of cones yet they found it necessary to create one that specifically for "thermal protection". I consider them the pros when it comes to cones, so that they felt it necessary to create a protectant clears it for me that you cannot, must NOT, skip protectants when using heat.