I have a 1959 Cessna 172 and have had the engine quit three (3) times while flying on a single tank.  To get the engine going again, I switched the fuel selector back to both or the other tank.  It seems to break a vacuumn lock in the fuel system as soon as you select another tank.  The last time it happened I was about 80 Miles west of the Alberquerque VOR in July 2003. (Not a friendly place for survival in the heat).  Needless to say, I don't switch to single tank operation at anytime.  If the fuel burn is uneven, it stays uneven and it will even out when I land and the airplane sits for a while.  I personally know of two other Cesssnas in the San Diego area that have had the same problem happen to them at least one time.

The  recommendation to switch to one tank was Cessnas cure-all for a fuel system problem.  I would personally use it with caution.