Do electricians often/frequently interchange the color on the grounded leg when wiring switches?
Not all grounded conductors are white, and not all white conductors are grounded conductors.

There's no problem using a white or gray wire as an ungrounded conductor, as long as the conductor is permanently reidentified.


  National Electrical Code 2014
  
  Chapter 2 Wiring and Protection
  
  Article 200 Use and Identification of Grounded Conductors
  
  200.7 Use of Insulation of a White or Gray Color or
  with Three Continuous White Stripes.
  
  (C) Circuits of 50 Volts or More. The use of insulation
  that is white or gray or that has three continuous white or gray
  stripes for other than a grounded conductor for circuits
  of 50 volts or more shall be permitted only as in (1)
  and (2).
  
  (1) If part of a cable assembly that has the insulation permanently
  reidentified to indicate its use as an ungrounded
  conductor by marking tape, painting, or other
  effective means at its termination and at each location
  where the conductor is visible and accessible. Identification
  shall encircle the insulation and shall be a color
  other than white, gray, or green. If used for single-pole,
  3-way or 4-way switch loops, the reidentified conductor
  with white or gray insulation or three continuous
  white or gray stripes shall be used only for the supply
  to the switch, but not as a return conductor from the
  switch to the outlet.


There is a problem with switching the grounded conductor, but no problem switching a white conductor (as long as it's reidentified).


  Chapter 4 Equipment for General Use
  
  Article 404 Switches
  
  404.2 Switch Connections.
  
  (B) Grounded Conductors. Switches or circuit breakers
  shall not disconnect the grounded conductor of a circuit.
  
  Exception: A switch or circuit breaker shall be permitted
  to disconnect a grounded circuit conductor where all circuit
  conductors are disconnected simultaneously, or where
  the device is arranged so that the grounded conductor cannot
  be disconnected until all the ungrounded conductors of
  the circuit have been disconnected.


Before the introduction of 404.2(C), two wire switch loops were common. Since a two wire cable only has a black and white wire (and ground sometimes), the white wire had to be reidentified and used as an ungrounded conductor.