Here in Mobile, AL on the Sunny (Soggy??) Gulf coast where we have about 80 inches of rain a year, rust and mildew (FWIW Not a part of this thread but keep an eye on your leather goods as well) are always a potential problem.
As far as my 550B goes, I have found that regular use (and lubrication to keep the press running) will prevent any rusting of consequence. I typically lubricate with BreakFree CLP which is great for keeping a coating of lube/rust preventitive on your press. However, I do not lube any part of my dies. IMHO, lube and primers/powder do not mix. Too much chance of contamination.
You can fight it, and somewhat reduce it, but metal in a humid atmosphere, unless coated in cosmoline will always rust!!!!
Just my humble opinion.
Dennis