Just to clarify the ApplyDST attribute - setting this by SSV is exactly the same as going to the Date/Time tab of the controller properties (when online), and checking the "Adjust for Daylight Saving (+0:00)" option.
Also, the amount of minutes that DST adjustment incurs is automatically selected by choosing the correct Time Zone.
All of this configuration is stored in the controller system values, and saved in the project file, so a subsequent download will restore the configuration.
So, in my opinion there is no need to SSV anything at all to the WallClock, except perhaps to re-synchronise the clock from a time-server, or "master" clock in the system.
Once you have made the configuration you want to work with on the Date/Time tab, those settings will persist.
Of course, if you want to programaticaly switch between DST adjustment and "standard" time in your application, or even to change the date/time to different locales, then it is perfectly OK to SSV this setting. If not, then you have code in your program that is using memory and scan resources, and it is not achieving anything.
However, if you want to guarantee these settings are made, then it is a good idea to put the SSVs into an intialisation subroutine called on first scan only (S:FS), or even use the dedicated Power-Up Handler in the Controller section of the organisor. That way the scan-time will not be compromised continuously.
HTH