Hello Roger,

I have done this many times ( because it works very well on getting a higher sound with 2 pins inverted ) and never had any problems.
The only difference is that I use a small resistor ( 100 ohms ) between the pin and the piezo ( one for each pin ).
The frequency depends on the piezo so it's best to test to see which is the optimal frequency ( as you did ).

Regards
Rui