If you plan to use the stock (factory) ROM, it is preferable to root. Otherwise, all the Custom ROMs out there already have root access.
The mentioned process is recommended to carry out using ClockworkMod Recovery.
- Download the latest SuperSU root package (Link: http://docs.google.com/open?id=0B46yaD9nFJdcUFV4RkJ2LVZPZmc)
- Power off your device and boot into the bootloader mode by pressing volume up + power buttons simultaneously until you see the bootloader screen. Scroll to "RECOVERY" using the volume buttons and select it using the power button.
- Now, you have Nexus S in recovery mode.
- Go to "mounts", scroll to the very last and select "mount USB storage".
- This mounts your device's storage to the PC you have connected to through the USB.
- Drag the downloaded "CWM-SuperSU-v0.98.zip" to the root of your storage.
- Select "unmount" to exit the USB storage mode.
- Go to "install" from the options provided in the recovery mode > "choose zip from sdcard" > select the "CWM-SuperSU-v0.99.zip" you placed in the storage and slide the bottom of the page flash it.
- Once flashed, select "reboot system now"