Long time ago I was taught that technically "ritenuto" is a temporary slow down internal to a piece, which should be followed by "a tempo" somewhere later to get you back up to speed. "Ritardando" would be generically slowing down, the opposite being "accelerando." "Rallentando" is supposed to be used for the slowing down (more or less, as in "molto" or "poco") specifically at the end of a piece.
Then you have "allargando" which is slowing down while getting louder, or at perhaps grander, and "calando" which is slowing down while getting softer or more gentle, these last two usually appearing at the end of a piece or perhaps at the beginning of the last section.
Not that anyone really cares... as said above, follow the conductor unless you are the conductor, in which case, do it however you please.
-Glenn