As a general rule, should input pins be protected with a series resistor?
The simple rule is that if the input pin is going to something that's 100% under your control and guaranteed not to misbehave, then no, you don't need protection.  If you can't guarantee that, then yes, you need protection.
The only time I'd advocate violating that principle is if, for some reason, you're designing an instrument whose measurements won't work right if you use protection (i.e., if you're building a $100 4-1/2-digit voltmeter, then it should be designed so that nothing can break it.  But if you're building a $100,000 10-digit voltmeter, then it can break if a grad student sneezes three feet away from it, as long as it says so in the manual).
Generally, &quot;under your control&quot; means that said input pin is inside the box, and it's going to something that you put in the box.  So if your pot or sensor are inside the product, and you can guarantee that they won't apply a voltage/current combination that exceeds the input pin's parameters, then you're fine.
If the input pin goes to something outside the box -- for example, if you're building a multimeter, or even a box that uses your bespoke cable with a sensor at the end of it -- then you should protect the input pins.