dearr mrs sewnsew (no relation) a good mechanical connection is desirable, as weil as the solder. Believe it or no, most solder doesn't conduct as well as copper to copper. The solder keeps things from coming undone and helps prevent corrosion and oxidization out of the joint. twisted together in a neat inline manner so the heat shrink can go over is what you are looking for usually. Twisted bare conductors under a wire nut is best left to house wiring and large appliances. Also there are some heatshrinks that are more resistant to wear. look at buyheatshrink.com for explanation of what types are available. Myself, i like using vintage style cotton over pvc insulated, twisted wire and replace all the old stuff.