The rotors that came with the brake setup for my rear where rusty when i got them. They didnt have any scale on the surface sweped out by the pads but it did have some surface rust. I sanded most of it off with some fine grit sand paper and the brakes seem to work fine but should i still have them turned to get a more uniform surface laid down or is it still ok to jsut let the rust wear off on its own?

 

As long as there aren't any grooves worn into the rotors it should be fine.

 

The problem with used rotors is you don't know if they are true. If they seem to work fine then they probably aren't warped, but it maybe a good idea to get them turned for a few bucks to make sure the surface runs true. Very light surface rust is not a problem. Just wipe it off. When it starts to pit the surface, then you must turn them.