You only need a pull-up or down to give a valid stable state when what normally drives that input is disconnected. This stops the program from seeing random input which may confuse it. When driving an input from another output you don't need a pull-up or pull-down.
You don't need pull-ups or pull-downs on floating pins which are not used, unless you are suffering interference problems or trying to achieve minimal current consumption.
Most pull-up or downs are usually 10K. Why ? No idea. There's probably some electrical explanaton somewhere, but it's a value which doesn't feel too low nor too high.