Proper Method of Powering Nokia 5110 LCD
According to the datasheet for the LCD controller it should run happily from 5V supply. The controller accepts logic levels up to Vdd, so I see no problem with that either. I also never had any issues with a Nokia 3310 display, which uses the same controller, operating at 5V. 

If you really want to run the LCD at 3.3V, then the proper way would be to use a level shifter (like 74HC4050) and external 3.3V voltage regulator.

I would be more concerned about the LCD driver implementation instead, here is a quote from the datasheet, page 15:


  Immediately following power-on, the contents of all internal
  registers and of the RAM are undefined. A nRES pulse
  must be applied. Attention should be paid to the
  possibility that the device may be damaged if not properly
  reset.
  
  ...
  
  The nRES input must be ≤0.3VDD when VDD reaches VDDmin
  (or higher) within a maximum time of 100 ms after VDD
  goes HIGH.


Therefore, be careful when implementing the driver and especially if you are planning on using someone else's implementation.