To the wing - Yes
To the flaps - never - I slide a piece of brass or aluminum tubing over it and glue in the tubing.  If you glue to the wing(or stab) at the center there is a possibility that the horn will not align absolutely perfectly with the hinges.  Using a tube allows some movement without sacrificing strength. Just my way, not the only way and if you are one of the few that builds everything absolutely straight it won't matter until you want to replace a flap and it is glued to the horn - yuck.

Ken