For your JNCIE-SP /-ENT or -SEC study you should always know which JunOS version is currently used in the track. This could give you a good insight, which features could be asked in compare to the exam topics. And of course, you should always use these versions in your own lab.
Here is short overview of the current (as of the date of this post) JunOS version in the practical exams:
|JNCIE-SP||10.4, changing to 12.3 in Aug 2013|
When you are browsing through Junipers website for configuration commands, you will find a drop-down menu at the top-right on the most pages. You can choose the JunOS Version you need and also compare with older or newer versions. For e.g.: 10.4 -> 12.3 maybe multiple syntax changes?…who know 😉
You can also find the always latest informations about JunOS versions in exams on the Juniper Website: