Official MT answer is always to use the latest available, if you are having issues then upgrade to latest available and then come back to complain. At the same time, the common wisdom is not to upgrade if you are happy with the current state of things.
Possible example why to avoid newer FW:
http://forum.mikrotik.com/t/new-ethernet-port-flap-issue-enquiery-pls-join/49200/28