i personally don't, unless the sealing surfaces are pitted or eroded.... but in this location it wouldn't end the world to use a lil permatex or rtv red / black / right stuff.... just don't get crazy. remember.. extruded sealer will clog the radiator.. AND you are gluing the cap and gasket on.. and the next fool that has to change it may cuss you :) ( and that next fool may be you ) :) BTDT thus i tend to plan on 'next time' when i make repairs..
if it is pitted or rough so that you think it wont seal. clean up the best you can with sand aper / wire wheel.. including the hose snout.. smear on thin and go with new gasket. same deal on hose snout. i have seen som soooo pitted they would not seal on a new hose.. thius a thin film of black rtv or similar to fill the potholes then push the hose on.. clamp, allow to cure, then circulate some water... cheack for floaters later. :)