You can edit the PCD8544 library to use any pins. Once I understood that 5110 uses SPI, I edited the library to use the Uno SPI bus. I couldn't understand why it wasn't set up for that in the first place. It shared the bus with an SD card without problems. I then moved the 5110 shield to a Mega without changing anything and later realised that it didn't seem to care if it was on the SPI bus or not. I didn't know till later that 5110 uses software SPI. I have now edited the library again using pins 8>12 on a Mega. I did this to make for a neater installation, but it may be what the library had in the first place.