How to drive 3 1/2 digit static TN LCD with ATmega328 in low power mode?
No, you don't need to refresh static LCDs anywhere near that fast. 60Hz should be fine- which will give you a couple years of battery life if it scales (it's normally in the range 30-100Hz). 

The main thing is that you need to make sure that the average DC voltage from each segment to backplane is very low (like &lt; 50mV). Easy when you're driving it with CMOS push-pull outputs. You must drive the COM high/low with a 50% duty cycle (very close to exact) and drive the 74HC595 outputs high/low in sync either inverted (on) or not (off). If you want to blank the display make sure no DC voltage appears across the display (the reason for this is that DC voltages cause electrochemical action which can permanently damage the display). It doesn't look like you are doing that. 

Here is an application note from Atmel that shows how to do it with a microcontroller, showing the proper waveforms: 



So probably the best way would be to wake up, flip the COM and RCK the new data in from the shift registers (perhaps alternate doing COM or RCK first), then calculate the next frame segment data, shift it out to the HC595s via SCK without transferring to the output register and go sleepy-bye. Repeat forever.