Well..there actually isn't a recommended releases page for those platforms (the only other one we have is for ScreenOS).
Why is a bit of a complicated question, but boils down to the amount of new code that went into Junos to support EX/SRX platforms when they were released and the fact that both those platforms are targeted for the the high-volume enterprise market.
We recommend reviewing the release-notes for the M/T/MX/TX platforms and when necessary contacting JTAC or your reseller for assistance.
HTH,
-Keith