Rosey,
I presently mow using a 30+ year old MMM. We had to redeck the mower after about the first 10-12 years because the original had rusted thru (Cleaned it off maybe once a year.) The "new" deck is doing very well as far as rust goes. I don't clean it off every time I mow, just every month or so and at end of season.
I just bought a BX22 + MMM. I'll be more careful with it. Where my PowerKing deck had mostly flat plate steel and was relatively easy to rebuild, the BX has a stamped deck with lots of curves, etc. I'm afraid if I let the maintenance go and lose the deck to rust in 10 years or so, it will not be easy to rebuild it. A replacement deck might not be readily available for a 10 year old tractor.... At that point, I know I could always get a RFM, but I'd rather keep the MMM around if possible. To me, it's one of those "ounce of prevention..." things.
UncleAl