If there is more current being dissapted by the resistor that it is design to withstand then it will eventually burn out and open, leaving you at some random time with a fan that stops working. Not good, yes?
You can place the resistor on either positive or ground, it does not matter because it is series (between source and fan). No matter where you add the resistor, current will go down, and therefore the voltage across the fan will likewise go down.