Elemental Mastery is in instant cast off of the GCD, so like trinkets will fire off on the first press of the button, no matter where you put it.

You could put it on the click macro, any of the called macros, or the fall through and the result will be the same.

The only things we control, are the order of spells which incur a GCD, and the amount of spam time (commas) until those spells are called again.