In most cases, the expense of an extended warranty isn't worth it for a crib mattress. You can reasonably expect a crib mattress made with quality materials by a reputable company to last for as long as your baby will need it. If you plan on converting the crib to a toddler bed, you may, however, want to look for a longer warranty with good conditions. The same goes if you think you might reuse the crib mattress for a subsequent child.