If I drive a piezo sensor with switched DC, will I get back over-voltage?
Yes, piezo elements can produce higher voltages than they are given when shut off fast.  To the driving circuit, there is a inductive component to the piezo's characteristics.  You deal with this the same way you deal with driving real inductors, which is with a reverse diode.  

Actually in the case of a piezo, it's a good idea to use two diodes, preferably Schottky, one each in reverse to power and ground.  That way, no matter what the piezo does as a result of mechanical ringing or getting externally whacked, the resulting voltage won't hurt anything.