Well, if you practice a lot and your practice includes scales on the wound strings, your nails will wear down. Unless you are super lucky to have a rock hard set. On the thumb, it may be peculiar to me, but mine wears down sometimes, not sure why. So I tend to practice with tape on it which alleviates the need to sand and smooth it, letting it actually grow out.
My friend who covers his whole finger tip is a scale monster, so he is always practicing across all six strings. It definitely saves your nails for performing while giving you freedom to practice as much as possible.
i've noticed when i did tape i, m and a, that it can rough up the treble strings, so some die hard players use a second guitar for this type of practice.
http://www.davidrussellguitar.com/index ... -the-nails