With most of the ready to run locomotives that also have DCC capability, the manufacturers like to use plastic clips to attach the wires to the board or decoder. To hard-wire it would be to remove the clips and attach the wires via solder and a soldering iron. It assures proper connection.
I've found that on some Athearns (the CF7s I have) the grounding wire that attaches to the frame via a screw is sometimes a little loose, even on new locos. While I have the shell off to attach the decoder, I verify that all wires are connected properly.
All my rolling stock gets Kadee couplers and metal wheelsets of the appropriate size (usually Intermountain) unless they have such in place. I also verify that the trucks are attached with screws tight enough so that one end (B end) usually has some up and down play (if the trucks are too tight, the car derails over imperfect trackwork, if the trucks are too loose, the car wobbles). I also verify weight, and add to it, if necessary. The majority of rolling stock I've bought has been correct, but occasionally an old car or a caboose will be a little light.
On older Athearn blue-box cars, as well as some Roundhouse, I've had to add grey or red shims (Kadee) to raise the car, since some of them tend to be low. I have not, though, solved the droopy coupler problem, where the coupler droops a little in the coupler box.
