If you plan on selling your iPhone or reselling it, then you should reset it, so as to clear it of all your personal information.
The quick and easy way to do this, is to connect it to iTunes and then using the restore function. (You will be prompted for backing up your phone, but as you plan on not using it – you can just answer no).
In addition, the under the Settings application on the iPhone, there is an option for resetting the phone. I am not sure if it is needed – but just to be on the safe side, I reset the device’s settings and content.