Well I was responding to your statement:
I guess I need to learn how to activate the internal pull up resistor, because I don't want to add resistors on every input!
I would state you really meant: I guess I need to learn how to activate the internal pull up resistor, because I don't want to add resistors on every input that I will be using!
So, yes pull-ups, either external or internal, are required in some cases, as in simple switch contacts. Other times they are not needed, and for sure if you are not using a input pin at all it does not have to be pulled up.
Lefty