i don't think it reduces earth damage specifically, just damage at the expense of mana. i've never notice it drain my mana though. i guess since they're off the gcd it's just switch as needed.
There are many fights where the boss goes invincible for a long time before doing undodgeable aoe, everytime I toggle fist of earth, then toggle back to fire.
Saves 10%, not much, but there is no opportunity cost.
I just redid all my Monk macros, and I think that you can get away with 4 major keys for all global cool down and have an optimal rotation.
Macro 1: FLANK
/ac "Snap Punch" <t>
/ac "Twin Snakes" <t>
/ac "Dragon Kick" <t>
Macro 2: BEHIND
/ac "Snap Punch" <t>
/ac "True Strike" <t>
/ac "Bootshine" <t>
Macro 3: DOT
/ac "Demolish" <t>
/ac "Touch of Death" <t>
Macro 3: AOE
/ac "Howling Fist" <t>
/ac "Rockbreaker" <t>
/ac "Twin Snakes" <t>
/ac "Arm of the Destroyer" <me>
So the way this works is, no matter what you do, even if you spam these macros wildly, if you are in range, you will always keep greased lightning up because if you can use snap punch or rockbreaker, or demolish you will.
The only downside is that you have to memorize which skill is on each macro for strange instances. For example if you are hitting a target that is facing you true strike is the best skill to use because of its high potency without a condition.
Also, on single targets, you will have to use the AOE macro to use howling fist, which is off the global cooldown.
the macro command
/macroicon "Howling Fist"
if added to your AOE macro allows you to see when it is off cooldown.
With this setup, the best rotation on things like ifrit is to hit the flank macro twice while at flank, then hit the dot macro twice, then move to behind and hit the behind macro 3 times, then flank 3 times repeating until the dots wear off.
Additionally, you should use howling fist as soon as you reach 3 stacks of greased lightning for max single target dps (because it is off the GCD).
Note that I have left Steel peak out, which I also use off the GCD for max single target dmg, I have this bound to my mouse as interrupts are split decisions, so it gets its own key.
Before I made this set up I was simply not using True Strike because it made for a clumsy set up, now I think I can easily have a theoretically perfect rotation with only a few keys.
Note that I just click my buffs such as internal release and blood for blood.