It would be best to get rid of it before seeding, otherwise you might be wasting your time. As you water the seed and apply starter fertilizer, the stiltgrass will happily take advantage of that as well and grow competing with the seedlings. If you have large infestation areas, glyphosate might be the best solution, it will kill the grass too and you can re-seed. If you have a lot of grass that you want to save, Acclaim herbicide will take out stiltgrass selectively.

Edit: there's pictures now. There's not much fescue to save, I'd hit it with glyphosate. Get on a good pre-emergent program for next year to prevent it from happening again.