I personally prefer to sand blast before repainting IF there are places where the previous paint is missing. I don't like to paint over missing paint areas as to my eye they always show through. But I also happen to own a sand blaster and it's great fun to use so ... I use it
That said, I have only done my old Singer treadles. None of them are "rare" or have any historical significance. If I acquired a rare machine that had irons in less than perfect state ... I may not touch them at all except for a gentle clean.