Do Mercury and Venus exhibit retrograde motion? This is more a terminology question, since Mercury and Venus certainly "move backwards" in the sky. The question is whether we call this retrograde motion. I'm referring to apparent retrograde motion here, not retrograde rotation.

I'm under the impression we don't, and reserve that name only for planets that are further away from the Sun than us; however conclusive evidence for whether this is (or isn't) the widely-accepted meaning for "retrograde motion" is hard to find.
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.