the general rule that I was told is this:
1. always replace the pads in pairs
2. The discs or rotors only need to be replaced if they are bad and the same goes for the calipers. Since rotors are installed new in pairs, if you have one side with more damage or uneven wear compared to the other side then you have another issue all together and might want to find the source of that issue.
I like to refresh everything at once on both sides... yeah it might not be not necessary but I am OCD like that and would feel the vehicle would be unbalanced or something silly like that :flipoff:
but if you have one worn out rotor go ahead and replace the damged unit - monitor both sides after a couple hundred miles for wear trends - and replace as necessary