It is recommended by some websites
I understand the "some" is because older Arduinos did not have sufficient power available on the 3v3 pin and those sites are outdated.
My 5110s all run off the 3v3 pin and no resistors anywhere except on the backlights. And therein lies a bit of a joke. The backlighting is quite different from my 5110 phone and is so useless that, for anywhere other than total darkness, you might as well leave it disconnected. This particularly applies to the blue backlights. The white ones are a lot better but still poor.
Here are some useful linkshttp://blog.stuartlewis.com/2011/02/12/scrolling-text-with-an-arduino-and-nokia-5110-screen/http://www.carlos-rodrigues.com/projects/pcd8544/http://code.google.com/p/pcd8544/