Bench syncing the carbs is just a rough sync... enough to make it run. The "real" sync comes from tuning so both sides pull equally. You can do that by ear and feel... or with a homemade or store bought manometer or sync gauges. So... set them (for idle) on the bench, install 'em and then do a proper sync.
Far as idle and WOT (wide open throttle) being off from each other... there's an argument to make in favor of biasing towards WOT. Being out of sync means one side is pulling stronger than the other (duh

). In a 360° twin, that means more vibration. I'd argue that syncing them at the throttle setting
you use the most is where you want to be. If most of your time is spent out on the highway at 60mph, why not sync the carbs so they pull evenly there? Of course, that means using sync gauges that you can tape to the handlebars and a deserted road you can (safely) make your runs on to check and adjust the sync.... but it's doable.
On the other hand, if all you do is putter around town... a bar hopper, setting for sync at idle would be your best bet.
If it's a race bike, set 'em for WOT.