When oscillating a piezo, should I use AC or DC square wave?
A sine wave is recommended because it consists of only one frequency.  A square wave consists of the fundamental frequency plus all odd harmonics.  When the piezoelectric stack is excited with a sine wave, it will vibrate only at that frequency.  When the stack is excited with a square wave, it will vibrate at the fundamental frequency and at all of the harmonics.  Some of those harmonics may be near mechanical resonant frequencies of the stack and can cause excessive and possibly damaging vibrations. Also, with all of those frequencies present, it can be difficult to interpret the results of the stack vibrating whatever is attached to your shaker.