It probably is _always_ the case. At least the very heavy burden of proof is on anyone who claims differently.
I would guess that Engels himself would never have made the claims for himself that so many "worldview" Marxists explicitly or implicitly make.
I have yet to see anyone on this list, incidentally, who tried to take seriously Postone's argument, which I find overwhelmingly probable, that Marx produced a Critique of Political Economy, NOT a Critical Political Economy. All the so-called "social sciences" are of relatively recent origin, and _none_ of them seems, to me, even potentially capable of becoming, as a discipline, coherent or systematic, let alone "scientific," in any sense of that word.
Robert Albritton has suggested the study of the current "economy" should be seen as history rather than a theoretical subject. Marx of course focused on an abstract capitalism. The contemporary capitalist economy is too embedded in contingency to be the object of systematic theory.
Carrol