You may not need this but if the nut is already off, you may want to check it for and bad grooves. If the strings are really biting into it, then you may want to replace it. Also, you can lower the action a little bit too if you want by filing down the bottom surface of the nut, slowly and resetting it to see how the action is. Basically, you can file down a pencil so it is completely flat on one side. Run if over the first and second frets and mark a line on the nut to see where the bottom of the strings are in relation to that line. If the string depth is above that line, then you can sand down the bottom a bit. If it's below that line then you should replace the nut.
Just my $0.02
Dave