Fair play for getting to this stage so early in the year after the wet weather we've being having!
So there are no weeds showing at the moment? AFAIK there is no point in spraying weedkiller if there are no weeds.
You have two options:
1/ Plant the grass and let it take its chances with whatever weeds will emerge. This way you will have a nice green lawn by May
2/ Wait a few weeks for the weeds to emerge, spray with weed killer and wait another few weeks for the weeds to die before you sow your grass.
This way you will have a nice green lawn in July.