Is it now a good time to buy rotating Magic Cards?
This answer is based on the very weak assumption that history will repeat itself.

When this answer was written, cards rotated once per year. Wizards of the Coast is moving to two rotations per year. This will drastically change the pattern described below.



Innistrad was the first set of a block that rotated in October 2013. The rares and mythics that are not seeing play in Modern or Legacy from that set trended towards an all time low around December 2013 before finally beginning to rise due to being out of print. 

If Return to Ravnica follows the same pattern, the rares and mythics from the set that are not seeing play in Modern or Legacy will trend towards an all time low around December 2014 before finally beginning to rise due to being out of print.

I used the price history line charts on TCGPlayer.com as a reference. I used my own experience to determine what cards see play in Modern and Legacy.