Utility is an economic term for a person's overall well-being and satisfaction. Consumers play a role in the economy by maximizing utility, which means allocating their scarce resources, such as limited time or money, most efficiently. Utility maximization is different for every person.
You can start using it right away but it was suggested to us by our midwives to hold off until 6 weeks as pacifier use can mess with their breastfeeding latch (it's a different kind of sucking).
Using a pacifier is a personal choice. Many breastfeeding mothers find they have no use for them, but there are also moms who have found them to be helpful. If you plan to nurse exclusively, it's important that nothing interfere with your milk supply -- especially during the first six months of baby's life.