I flew the Q400 as well as the 200 and 300, and was always told that we select the boots on as soon as we detect icing. Some people used to mention the bridging theory and that you should wait for ice to build up, but the reponse was always: "That's what the dwell time is for."
So I was always told that the bridging theory, whether or not it is possible on other types (and the evidence appears to be that is it not, or at least no factor), it is not relevent for the Dash de-icing system.