Subject to any unique engine characteristics, carb heat should be selected before closing the throttle because by doing so you are utlising more "heat" from the engine as it is then running at significant power setting.
On a glide descent the engine should be "cleared" every 1,000 ft or so, to purge the plugs of any lead deposits and also check for normal engine operation. Before the engine is "cleared" carb heat should be selected off and then selected on again before closing the throttle.
If you stick to this procedure you should stay out of trouble on a glide descent.