Is it generally OK to ignore the current in and out of a MCU pin when analyzing circuit external to the MCU?
Maybe. For values in K ohms usually you can ignore it, unless you're doing something with unusual operating environment or high reliability. It never hurts to read the datasheet thoroughly when you are considering spec'ing a new part though.
Of course if you enable internal pull-ups or pull-downs then you will generally have to take those into account.
If you try to minimize switch-closed consumption by using a resistor value like 10M then you will find that the MCU maker won't necessarily guarantee that will work.
For example, the Microchip ATMega328p has the following guaranteed characteristics:

1uA of leakage is enough to overcome the few hundred nA from the 10M resistor, so it might not work. Chances are the current will be much lower at Ta &lt;&lt; 105°C so it might be okay for a toy or consumer device.
There are reasons not to use too low a resistor value (wasted power, possibly switch wear in extreme situations) and too high a value (switch reliability, leakage due to the semiconductors or contamination on the PCB and possible pickup of EMI).