Prudence - Never underestimate the opponent Quotes

Is this applicable to all opponents in one's life?  Bad habits, tendencies, greed, people around you, difficult situations... 

Never underestimate - Underestimation can be fatal.

From the Tao Te Ching, Chapter 69

In using the military, there is a saying:
I dare not be the host, but prefer to be the guest
I dare not advance an inch, but prefer to withdraw a foot

This is called marching in formation without formation
Raising arms without arms
Grappling enemies without enemies
Holding weapons without weapons
There is no greater disaster than to underestimate the enemy
Underestimating the enemy almost made me lose my treasures

So when evenly matched armies meet
The side that is compassionate shall win

Derek Lin Translation

A master of the art of war has said, 'I do not dare to be the

host (to commence the war); I prefer to be the guest (to act on the
defensive). I do not dare to advance an inch; I prefer to retire a
foot.' This is called marshalling the ranks where there are no ranks;
baring the arms (to fight) where there are no arms to bare; grasping
the weapon where there is no weapon to grasp; advancing against the enemy where there is no enemy.

There is no calamity greater than lightly engaging in war. To do
that is near losing (the gentleness) which is so precious. Thus it is
that when opposing weapons are (actually) crossed, he who deplores (the situation) conquers.

James Legge Translation