Hi Mirage_Man,
There is no fixed “rule” here because there are many different types of innersprings and many mattress designs that can be built around each of them. You can read more about box springs vs a firm non flexing foundation in post #2 here.
If you were being told this by two different manufacturers who make their own mattresses (not just salespeople at a store who may not know) then it would probably be safe to say that they are both correct when it comes to the specific design of their own mattresses and I would follow their individual recommendations.
Phoenix