What is the Biblical basis for the concept of 'balance' between 'two extremes'? Any Christian at some point will be labeled an extremist. Also every Christian at some point will call other Christians extremists.
Even in any given local church, some will label each other this way.

The question is from what Biblical basis can we say 'balance is good', and therefore what kind of extremes should a person avoid?

For example, most Christians would agree that never reading God’s word but just being blown around by the Spirit would be an extreme position. In the same vein, always reading God’s word but denying the importance of the Spirit for understanding the Word would be the opposite extreme. Is this Biblical for me to paint extremes as bad things this way?

What is the scriptural basis for balance and what might be some commonly held views that are considered 'extreme' by most Christians, regardless of denomination?
Ecclesiastes 7 is even more direct:


  In this meaningless life of mine I have seen both of these:
  the righteous perishing in their righteousness,
      and the wicked living long in their wickedness. 
  16 Do not be overrighteous,
      neither be overwise—
      why destroy yourself?
  17 Do not be overwicked,
      and do not be a fool—
      why die before your time? 
  18 It is good to grasp the one
      and not let go of the other.
      Whoever fears God will avoid all extremes.


That said, the Golden Mean is primarily Aristotelean in origin. That alone does not exclude it from Christianity, becuase much of Aristoltelean thought was incorporated into the Church's doctrine in the first few centuries of the early church.

Galileo, for example, was really having quibbles with Aristotlean cosmology, moreso than anything in Scripture itself.  The notions of perfect spheres and earth-centricity are not found in Scripture, but had been accepted as dogma by the Church, and as such were defended.