If you use a crab grass preventer (an herbicide to stop the germination of crab grass), it will also stop the germination of any other seeds you sow. Some products recommend waiting 3 or 4 weeks before seeding, others suggest 8 weeks. In any case, there is no point in seeding your lawn and then applying a product that prevents germination. You would be wise to wait until fall to reseed if you plan to apply an herbicide this spring. Otherwise seed this spring and use the herbicide in fall.