If you don't lay it down in the gap as Mike showed you it will hinder the movement of the control surface & that is exactly what you don't want!!

Many 3D Foamy planes are hinged this way. Instead of using hinges the kit comes with clear "hinging tape" so that you are hinging & sealing the surface all in one step.

I haven't been bothering to seal my hinge surfaces either. I was going to until I realized that I get all the response I need just the way they are with allmost zero gap to speak of, just enough to allow full deflection. I suppose if I ever became a good enough RC pilot that I was setting up a real serious 3D plane I would go ahead & do it but for the kind of flying that I am capable of, (just sport flying around for fun) my planes don't really need it.