This is a really practical tip when preparing your baby layette. Just buy enough baby clothes to last your child for a week without doing laundry, with two or three nicer ones for going out.
First of all, since newborns have sensitive skin, it is recommended that their clothes, especially those soiled by spit ups and poop, are washed as soon as possible. Other than those, however, the clothes basically remain clean as newborns are clean and thus won't require intense washing. So they're also easier to wash and wear.
Newborns really balloon in the first three months or so, which won't allow you much mileage on newborn-sized clothes. I bought a few 0-3 and more 0-6 months clothes and only two newborn-sized shirts.
And then, there are the gifts from friends and family, just because they're blessing your baby, attending your shower or joining your family in the baby's dedication/christening. Not to mention hand-me-down clothes as well.
So limit your newborn-sized purchases and channel the savings on more important items instead, like a fab sling or a sturdy pen.