I would definitely not add the key just to make the patch available on the machine. Pretty sure that is how you can BSOD your machine.
For Symantec once I ran live update the registry key was created and I was able to apply the Microsoft patch, before the update the registry key did not exist. I set an analysis looking for that key and had symantec run live update locally on the machine and as they report back they now have the key as well.
After applying the Microsoft patch I am protected for Meltdown now based on the Powershell command but still vulnerable to Spectre until I update my BIOS.
Analysis I used to check if the key exists
exists key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\QualityCompat" of native registry