I have a selection of engineered modules stored away for future use, but only smaller, non-essential ones: multiple DSS with extended range, some missile racks with high capacity, lightweight limpet controllers, that sort of thing. Most of these modules have only one sensible engineering choice anyway, so the choice is not dependent on a specific thing I want to do in the future. That way, I don't have to pin these blueprints and I can use the engineers' blueprint slot for something else.
Like the others have said, I wouldn't stockpile engineered core modules, as the engineering choice depends heavily on the particular ship and the task you want to do with it. You could store a few class 5 FSDs with engineered Extended Range & Mass Manager, as we have lots of ships using a class 5 FSD (I counted 14), and a higher jump range is usually the most useful choice for most endeavors. But then again, with a pinned blueprint and Felicity Farseer within easy reach, why bother.