Hi,
The purpose of a Pull Up resistor, or a pull down, is basically to prevent that i/p pin being left floating / open circuit by an input device.
If that happens then the micro will sense all sorts of random voltages.
Its therefor generally good practice to pull up or down all Digital inputs, however the resistor values should not be that low that they could affect the input device.
Typically 10k is a good value for either, which one depends on your softwares logic, ie pull up means the default state of the pin will be 1.
On PortB you have an option to select and turn on an internal pull up resistor.