Yes, the only ABSOLUTE hidden skill you need to learn, is the Ending Blow, since you're on your way to the Forest Temple, and the Golden Wolf blocks your way, there's pretty much nothing you can do about it, only to let it take you to the ghostly realm where you learn the skills. The other skills, however, are learned with Howling Stones. Means, you need to howl a specific song at the stone, and then the Golden Wolf will appear, and allows you to learn the other skills. So, yes, the other skills are completely optional, but will make the game a lot more easier to beat. For an example, mastering the Back Slice will prove most useful against Dark Nuts, and enemies that tend to block frontal attacks, or have a shield of sorts.