Back when cars had carburetors, hard acceleration would reduce fuel efficiency, because the accelerator pump in the carburetor would squirt extra fuel into the carburetor each time the accelerator was depressed. You could see the wasted fuel blow out of the exhaust pipe as blaclk smoke.
With modern fuel injected cars, that does not happen. Instead, the computer constantly adjusts the fuel mixture to the optimum ratio, depending on engine speed and throttle setting. So I really don't think it matters a whole lot.
A couple of years ago I read an article that suggested, based on computer models, that is was better to accelerate the fuel injected car up to cruising speed relatively quickly (not flooring it!), because that would mean less time spent with a richer fuel/air mixture and thus less total fuel expended. But that was a computer simulation, not real world driving.
Elliot Richmond Freelance Science Writer and Editor