You have to decide on a case-by-case basis on whether or not to use Fluid Aura. If the enemy is knock-back immune, sure you can use Fluid Aura as much as you want. However if the enemy can be knocked back by Fluid Aura, make sure the enemy you push away will die soon.

If your party is trying to AoE a group of mobs and you Fluid Aura an enemy that can be knocked away, your 150 potency Fluid Aura is a potency loss for your group. If you happen to knock away the target your Black Mage was targeting with Flare, that is a severe DPS loss. You can cause caster DPS to be line of sighted (LoS) which a large DPS loss. If you knock away an enemy, all melee jobs will lose DPS.

Fluid Aura can be helpful for mechanics such as interrupting casts, but you have to weigh the pros and cons. For example, using Fluid Aura to push the Green Golem in Second Coil of Bahamut Turn 4 raid to interrupt his AoE Demolish cast is good. However, using Fluid Aura in Midas Savage A5S to push the pigs back into the Minotaur will cause a party wipe.