The expression of the grain of salt dates back to the roman who used it to express many different meanings, it started its meaning as an anti-dote, then it was used to describe a powerful poison that only required a grain and finally it also described caution in general. Today you will hear that all frameworks, principles or virtually everything in architecture should be used with a grain of salt, making everything relative.
The strange thing is that even if we use it we can never agree on what it actually means. For some it is wisdom, caution, life experience, laziness, politics and/or something else. So the important thing is always to clearly define what people are meaning when they talk about a grain of salt, unless the whole architecture becomes meaningless.