"It is a bad idea to keep it on the dash and a worse idea to move to the dash during a traffic stop."
Common sense alone has me agreeing with that.
The officer pulls up behind and watches carefully for any possible sign of hostile movement. And you've just visibly moved a handgun to within your reach on the dashboard. You've escalated a traffic stop to a possible life-threatening encounter.
Best bet. Leave it where it is. Keep your hands in the officer's view. Tell him where it is and ask for instructions.