Wirelessly posted (BlackBerry8703e/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/104)
You can purchase or add value to a Metrocard in 5-cent increments. I've never had a problem doing so. (There is, however, a $4.00 minimum for purchasing a new pay-per-ride Metrocard; I'm not sure if there is a minimum for adding value to an existing card.)