Re: new lawn
If you're going to rototill the soil, then there is no point in spraying herbicide because you're only going to kill what's visible. When you till, you'll be bringing new weed seed to the surface where it's warmer, moister, and prime for germination. If you've got a weed problem, I would recommend laying sod rather than spreading seed. You can apply a pre-emergent before laying the sod that will help to keep new weeds from germinating.
Other benefits of sod is that it's "instant" green, you can walk on it immediately (with care ), it requires less water and aftercare than seed, and provides a thick blanket that will smother out most weeds and prevent new ones from germinating.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!