I successfully flat iron my roots every week after a rollerset.
i have no sign of damage. and i've been doing it every single week since september. And my ends are still entact.
just make sure u use a heat protectant. And if ur too worried about ur ends, don't go over the entire length of hair.
I'll admit i do it ever so often, but mostly i'll just do the roots.
If i'm really far into a stretch and my roots are craaazy. I'll prepoo with conditioner w/ a lot of honey. I'll apply it only to roots and it has helped me loosen the curl so when i'm ready to flat iron, i just have to go over the hair once and it's straight! even with a bad rollerset.