Do I have to take what you say with a pinch of salt? In other words, can I trust what you say? Or is it unpalatable and needs some seasoning.
American English apparently favours a “grain” of salt, rather than a “pinch”. Pinch is better, I think; it’s got that kinaesthetic edge to it and conjures up so many more images. You can picture me actually putting thumb and forefinger together, picking up some salt and sprinkling it on the situation (as above).
The OED defines the phrase as “to regard something as exaggerated; believe only part of something”. Its origin seems to be culinary or even from a recipe for an antidote to poison! For sure, most things taste better with a pinch of salt, though if the ingredients are of the highest quality it may be unnecessary.
