Using VU mode button when using driver hours books

It may be driver café talk but when using a driver hours book on domestic rules (utility based work) do you also need to show a break on the tachograph unit by changing the mode button even though there is no driver card inserted in the VU?

Some drivers are concerned that the DVSA are checking the VU’s to see if the mode switch has been used.

As far as I’m aware there’s no requirement to use the tachograph mode switch at-all if you’re using log books on domestic regulations, I don’t see why the DVSA would be interested in seeing anything other than the log book for periods that the driver is on domestic regulations, I suppose it’s feasible that they may compare what the tachographs show to what the log books show but that could really only be driving time time.

Having said that I don’t see being on domestic regulations as a reason for drivers not to have proper breaks, after-all a tired or fatigued driver is not a safe driver.