alex,

Ed is right, either take it off or leave it on.
Ive always left it on in the past and just soldered underneath, and then youll have to cut some slots in the outer casing to allow the wires to come out the box.

saying that if your wires are coming out, theres most likely a reason, give them some more slack,
I dont normally tin the wires as i find it causes them to come out, and just twist the ends and make sure the wire is all the way in before tightening.

John