Yep, but generally you have a good idea of how fast things are dropping.
You start casting Bio II or Miasma and by the time you finish, its already at 75% hp? Drop the second dot, immediately bio then Ruin II
You finish 1 dot and its close to 50%? Ruin + Ruin II.
Since generally melee combo is strongest on last hit, if you have a good sense (based on animation, the last hit has a huge flashy animation, or if you play those classes, you'll know) you can also tell when a mob is going to spike down.
The actual dps difference is marginal - without a doubt if an add is blowing up we're going to suck hard on it, but if you want to optimize you could.
And yea I realized I was confused on Miasma and Bio II, just swap the two in my OP.