Do Mercury and Venus exhibit retrograde motion?
As noted by PM2Ring, the notion of "retrograde motion" predates the "Copernican revolution" So you should think about the motions of the planets from a Ptolemaic view:


  The planets have two major motions (in addition to the diurnal motion around the Earth) motion with a period of one year, and a second motion with a period that is specific to each planet. For Mars, Jupiter and Saturn, the main orbit has a long period, and the epicycle has a period of one year.  For Mercury and Venus, the main orbit has a period of a year (keeping them aligned with the sun) and the epicycle has a period of less than a year.


When the right ascension of the planet is decreasing, then we say it's motion is retrograde. Retrograde periods occur during inferior conjunctions of the planet with the sun, and so the planet cannot be easily observed while it is retrograde.