I just got MM (my first Zelda game by the way. Played a bit of OoT before, but not much) recently due to my PS3 currently being repaired, and so far...
The Inverted Song of Time has hands down been the most useful thing in my arsenal, making the short three-day timer feel more like a week and greatly extending how much I can accomplish before the inevitable reset with the regular Song of Time. Add to this the Song of Double Time, which skips time ahead by twelve hours and is akin to Oblivion's "wait" feature, and I frankly just don't see any possible downside to using the Inverted Song at all times. Is there any reason what-so-ever to not use this every possible moment in the game? I'm sure not seeing one.
I've beaten the Swamp Temple and am preparing to head off towards the Mountains I believe.