Mowing and Fertilizing Newly Seeded Lawns

Seeded Easter weekend; grass coming up with weeds. When to mow? The expert says mow when grass reaches 1.5x mower height (e.g., 6" for 4" max). Follow-up: last fertilized Feb with winterizer; plans to use Scotts Turf Builder with weed control. The expert advises waiting until grass is firmly rooted (month+), using organic, low-solubility fertilizer like Milorganite.