The March 6 update to the HBD engine features some important developments we should all be aware of. mnnorthstars mentioned one of these in the chat page: Spring Training playing time now has increased importance. Specifically, those with adequate playing time could get a friendlier boost in ratings, and big league players who don't get enough playing time can take a hit in their ratings. This latter element is the key thing here to note.
As far as what equals adequate( or as tzentmeyer called it "enough") playing time, we don't know yet. Since this update is so recent, there have yet to be any studies in the forum. Before the update, I tried to get as many players as possible 40 at bats and 15-20 innings for starting pitchers to capture the Spring ratings boost.
Perhaps when ST is over some of us can report back here with the varying results of playing time.
The other update to the engine involes rookies and big league service time. Previously, you could call up rookies roughly 12-15 games into the season and avoid them garnering enough big league service days for their first big league season, effectively giving you an extra year with those cheap youngsters. Now, it sounds like you'll want to wait around 20-25 games for the same effect. We can call this the Evan Longoria effect, since that is clearly what the Rays(no longer the Devil Rays, though) are doing with him this season.