As everything in economics, it depends on which branch you're talking about, but this assumption is the core of the marginalist revolution which is fundamental to the whole classical branch and it's derivatives, which makes the vast majority of mainstream economics nowadays, because the neoclassical synthesis made the biggest part of the Keynesian school move toward this assumption (say hello to micro-founded macro).

Anecdotally, claims that [insert a foundational hypothesis of classical economics] isn't really that important in the whole model has been the favorite defense of neoclassical economics against criticism since at least Friedman in 53 ( it might have existed before, but none of them had the popularity of his Essays in positive economics)