In understanding the universe we try to avoid those, but we all must make some. A wild-ass guess need not be a deemed a "fantasy." Science works with the assumption that nothing travels faster than the speed of light in a vaccuum. As far as I am concerned, that is a fairly large assumption - large enough to be a wild-ass guess but not a fantasy insofar as reason has been able apprehend reality. If we were to work with the assumption that nothing moves faster than the speed Jeff Gordon in a dump truck, then we would be engaging in a wild-ass guess that could be deemed fantasy.
Science doesn't 'work with that assumption'; science reaches that conclusion. There's a major difference.
Ballocks. That's a deduction, not an assumption.