General rule is that CPS can be applied with very minimal testing, and M-release much more testing. To M020, we have applied CPS03, CPS06, and now CPS09.
For customization, you should be using the wtSafeArea. Reference the Customization Guide for more information about this. For any changes to your modified files, you can look to see if PTC has updated your file or not with the CPS.
In reference to your note about M030, please know that PTC updated their M030 "read this first" document on Friday. If you're running M020 CPS09, then you cannot upgrade to M030 until CPS02 is released, probably at the start of April.