What is the basis of the LDS/Mormon prohibition of facial hair for men? I have read in a few places the LDS Church has at least a general practice, if not an official teaching, that prohibits men from growing facial hair--at least for men in any position of leadership.  Apparently, Joseph Smith was "clean shaving", but many of the other prophets and leaders in times past did have significant beards and moustaches.

So, my question concerns what the basis is for this teaching and when that basis came about.  Is it an official church teaching or an unwritten policy, and is there any reason given for why facial hair, which God seems to have intended for men in some way, is inappropriate or wrong?
Short version: The rule is about appearing nice, clean-cut, and respectable in line with current cultural norms.  It's a matter of practicality - just look nice, in order to better represent the Church and therefore, God.

From an address President Dallin H. Oaks gave to the 25,000 students of Brigham Young University. 


  Unlike modesty, which is an eternal value in the sense of rightness or
  wrongness in the eyes of God, our rules against beards and long hair
  are contemporary and pragmatic. They are responsive to conditions and
  attitudes in our own society at this particular point in time.
  Historical precedents are worthless in this area. The rules are
  subject to change, and I would be surprised if they were not changed
  at some time in the future. But the rules are with us now, and it is
  therefore important to understand the reasoning behind them.
  
  In the minds of most people at this time, the beard and long hair are
  associated with protest, revolution, and rebellion against authority.
  They are also symbols of the hippie and drug culture. Persons who wear
  beards or long hair, whether they desire it or not, may identify
  themselves with or emulate and honor the drug culture or the extreme
  practices of those who have made slovenly appearance a badge of
  protest and dissent. In addition, unkemptness—which is often (though
  not always) associated with beards and long hair—is a mark of
  indifference toward the best in life.