You want to clip the wires to the arm. You just want to make sure the arm can move freely without causing the wires to pull and break. Before putting the drum on swing the are to the rear and check the wires at that point. You will be able to move the arm more than it will ever move within the drum and that should give you a good indication you have enough slack in the wires.
As for using a rubber band to hold the magnets before installing the drums I have to say I might agree with you mechanic. While I have never installed brakes without the clip that limits the spring travel I would consider it in the future. The reason I say this is the cheap Chinese magnets now used give off dust that will act as cement and bind the magnet to the arm limiting the total travel of the magnet and in time you will lose all brakes. The rubber band will break on the first application of the brakes and fall off.
I had this problem 2 years ago when I lost all 6 brakes on the way to Fl. Pulled the drums and found the magnets cemented in place by the dust. The removal of the clip might keep this from happening again as there would be more clearance between the magnet and the arm.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles