The answer is it depends ..............
You don't have to, as you can turn the crank which in turn moves the cams which in turn lets you measure the clearances.
You can take them off; then you can rotate the cam independently from the crank but that does have some problems if you don't know what your doing.
Have a look at the video's posted on the site and it is quite clear what the "recommended" method is.
For me, why take the belt off if you don't have to?
I've owned dozens of bikes over the years and my Duc is one of the easiest to maintain. Example valve clearances:
SV650
You need to remove the fairings then dump the water to get to the rocker covers. Then you need to measure the clearances and if they need adjusting you need to remove the cams (including the cam drive) remove the shims then re-assemble to re-check. If you screwed it you start all over again.
On the Duc, I simply measure and if adjustment is needed I "flick" the retainer out of the way, move the rocker arm and take out the shim............soo easy
Just my two bobs worth.