I would be very surprised if 10% of this forum's user utilize any kind of attached locking device on their handguns. The point I'm trying to make is don't let a cable lock ,or any kind of lock, deter you from proper handgun storage and safety.
The first thing I wanted to say was scrap a cable lock and get a gunvault or some kind of mini-safe, but they can easily run you $100+. If that's not an option, a cable lock is going to have to do (although you nearly eliminate any kind of response in a home defense scenario...this is something to think about).
Depending on how often you are willing to take out the firearm for a quick "inspection," i.e. often enough (once a month or so), keeping the slide open or closed really shouldn't matter. If I were to use a cable lock I'd also avoid all unnecessary wear on the spring and would slowly close the slide on the shielded cable for prolonged storage. Like yours, most are covered by nylon/rubber/plastic so there is little worry to harm the finish around the chamber.
Let the slide move forward slowly as to not bite through the cable coating. To do otherwise, cable or not, is just bad practice. Letting a slide slam on an empty chamber does take its toll on a firearm.