Suppose you're playing a non-permission strategy such as Zoo. You decide to run Mental Misstep because of a healthy paranoia of losing to combo. You enter a tournament and start round 1, losing the coin flip to decide to go first. Your opponent starts with Tundra, and says go.

You have a Wild Nacatl and a Mental Misstep in your hand. You chose to cast the creature, and face a Mental Misstep paid with it's alternate cost (2 life).

Do you counter the Mental Misstep?
What if you were on the play?

In general, when are appropriate times to hold back Mental Misstep? Should you use the counter aggressively in strategies that do not run additional counters (Daze, FoW, Counterspell, Pierce, or Snare)?