PG,
I referred that junior and senior years as being the most important, and I do beleive you confirmed that with Grienke's HS experience.
IMO, that is the most important time in the recruiting and draft process. These days we hear about pitchers doing more before that time, camps, showcases, tournies, when IMO not necessary, I am talking about pitching.
There is plenty more to come after the HS experience when it counts more, so I always suggest watch those younger years before it really counts.
Short shut down periods can work, but one has to know how to manage them, shutting down for two weeks doesn't mean you pick up the ball and pitch after 2 weeks rest, but I think that's understood by most of us but I'll bet that there are many who do not follow that, because they don't know or understand.
Management is the key and this can begin when a player shows promise as a pitcher by end of freshman or sophmore year in HS.