"True, but does it hurt if I do apply it? "
Grass is stressed by weed killer. That's one of the reasons why they
tell you to be careful to not over apply it, don't use it in very hot
weather, use less on certain species of grass, etc. I certainly
wouldn't use it on new sod that's just been planted and has no weeds.
Plus using unnecessary chemicals doesn't help the environment, as they
can wind up in ponds, drinking water, animals, or your kid's bare feet.
. For a lawn with only a modest amount of weeds, a small tank sprayer
is much more effective. It delivers the herbicide where you need it.
If it's new sod, did you use starter fertilizer when it was put down?
If so, and it's only been 3 weeks you don't need more fertilizer. If
you didn't put it down then, I would apply a starter fertilizer now.
It has more phosphorus (2nd number in the X-X-X percentage), which
promotes root growth.