Formerly the language around the CUs were fairly clear: CUs addressed specific issues, and if you weren’t experiencing that issue, then you shouldn’t install that CU.
Well, that was then and this is now.
Microsoft is now saying that you should install all CUs when they become available, and says that they undergo the same testing as complete Service Packs (SPs). So now instead of waiting for an SP, even if you weren’t having problems, you should also keep current on CUs.
One good way of keeping up with the release of both CUs and SPs is Brent Ozar’s web site, SQL Server Updates., which tells me that SQL Server 2014 is up to CU5 for SP1. On this site you can sign up for notification emails when new updates are available. It lists versions going back to 2005, I expect that might fall off the list in three years when 2008 begins falling out of support. (Microsoft says that when CU6 for 2014 SP1 comes out that CU5 will no longer be available for download)
So if you’ll excuse me, I have a couple of servers to patch.