My stretching tool of choice is the blowdryer. Because the blowdryer allows me to control the amount of heat being directed (by holding it far away and/or using the warm and cool settings). I get good results and straight managable roots simply by holding my hair taut (with my hands, not a roundbrush) So that the roundbrush won't be putting stress on my hair. I'll blow my roots until they are totally dry for they wont revert and then i'll quickly blow out the length of my hair (excluding the ends).
I don't use a pressing comb, because no matter how careful you are...you still would more than likely end up pressing a few inches of your relaxed hair. Which over time would lead to extremely dry hair. I don't use the flat-iron because you can't get to the very roots of your hair (without burning yourself). plus both of these methods are direct heat.
just my humble opinion....i'm sure all 3 methods work just as well...