Active rusting should always be removed. Stains and patina are up to the sword owner whether to remove or not. I use flitz metal polish most times and for severe rusting I use wd40 and a fine steel wool or brass brush. I clean the surface with alcohol to remove any petroleum distilates left and then use rennaisance wax ( a microcrystaline not a parafin wax that is used by the royal armory ). There are many other ways just as good though and you need to as Jen said read more and decide
here is a good link on preservation
http://swordforum.com/articles/ams/conservation.php