halving ISO speed?

I've recently begun shooting b+w film. I read in a book, Perfect Exposure, that by halving the ISO of a film, the negatives would be more accurately exposed. This was suggested for cameras with reflected light meters. Why? Also, assuming that this is true, would it still be recommended to spot meter off of the shadows and give two or three stops less exposure. Would I be messing around with the exposure too much?

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