To me, pretty much suggests pretty much of the time. So it doesn't strike me as exactly right for I'm pretty much sure of that. On the other hand, it's not that bad either.
Thank you, I think I understand, it could be that "pretty much" sounds better than "pretty" when referring to ungradable adjectives (interchangeable, impossible...), and "pretty" is better used with gradable adjectives (sure, good...). Thanks