Along with Letstalkpitching.com, I read another website called insidepitching.com. They have some pretty interesting articles on offseason conditioning/rest periods at different universities (LSU, Vanderbilt, Purdue, etc.) posted right now. These articles are written by the pitching & head coaches and provides a summary of what they do in the offseason. From what I've read it seems pretty much pitcher specific if you take a rest period or not & really involves a good deal of common sense and understanding of your past pitching demands/activities and a sense of what your body is telling you.
My opinion (take if for what it is worth) which is adopted from what my pitching coach has told me after I asked him whether I should shut my arm down before now because I have tryouts in early Feb and didn't want to go into them with a tired arm.
His rules were -
If you arm feels tired or seems like it is taking a longer time to recover from a throwing session than normal - then you need to rest some - skip a throwing session or two & let it rest up and see how it feels. If it is still tired after one or two of your throwing sessions you should probably shut it down for a little while. Of course this is a problem if your arm wears out the day before tryouts & you go into the season with a tired arm, but he said to vary your workouts/intensities and this would help keep your arm from getting tired/overused.
The address for the website is -