If its actually pitted from rust, I will most likely take it all off, or most of it to make sure I have removed the rust at the bottom of the pits so it doesn't continue to be active and eat deeper. Depending on the blade stock thickness, or how much blade is left on an older knife, I might just re-grind to below the pitting and start fresh; depends on the knife, and its look and feel.
A little red rust over patina, I just take the red off.
Some folks, mainly the hardcore collectors will really b*tch about any cleaning of a patina from a knife, as it ruins a lot of collector value of some pieces.
G.