There are a couple stipulations I've frequently heard of, for showers for subsequent babies to be considered "okay". If there are more than 5-6 years between children (well past the timeframe where you'd keep your baby stuff), then a shower is okay. If baby #1 is a boy and baby #2 is a girl, it's okay.
Another thing to keep in mind for subsequent babies is a "Meet the Baby" party. It's done sometime after the baby is born, generally within the first month. It's pretty much a baby shower, but with the added benefit of knowing with 100% certainty that the baby is a boy or girl. :P