Hi there Pluk,
The piezo element will contain some capacitance that has to be driven. This usually requires a lower than usual impedance. Depending on your capacitance a higher than optimal impedance would cause distortion of the square wave. You could look up the effect of a low pass filter on a square wave to find out what kind of distortion to expect. Since im not familiar with your application i cant say how much degradation from a perfect square wave your app can stand, i guess you'll have to determine that. If it seems like it might be a problem, then you would have to go with a symmetrical driver rather than a single transistor driver. If you arent sure, i guess you can try it and see if you get the results you are expecting.
A regular transistor with a collector resistor constitutes a square wave 'amplifier', where the smaller input can drive a much larger output. The collector resistor has to be small enough to fully drive the capacitance. The trade off is the smaller the resistor the more waste power, but the larger the resistor the less current drive is available for that capacitance.
A symmetrical driver would have a transistor on top and on bottom (known as a half bridge). There is a delay introduced between turning one transistor off and the other on so that they both cant turn on at the same time even for a tiny fraction of the cycle. This kind of drive provides very low output impedance for the load with little waste.
Do you really need 100 volts?
You stated that you are using this to drive a liquid out of a nozzle. I was thinking of doing something like this too for an entirely different reason. What i wanted to ask you was are you using any kind of check valve or do you find you dont need one?
If you are using a check valve, how are you constructing it?