I think what I've found is this:

If you only need slight, gradual accelerations, then do it carefully and stay in eco mode.

But if you need to pack on some MPH, then give it at least 50% - 75% gas, come out of eco mode, get it over with and get up to speed quickly and get back into eco mode. During heavy acceleration like that, you'll almost always get an IMA boost too.

I seem to get better MPG that way.