I currently run 105 (5700) on my bike. All stock cables from Giant.
I don't have any existing problems but I've read a lot about how great different groups shift and how different cables change the characteristics of a group.

I've read that Gore or Yokozuna are the best cables. I'm considering buying the Yokozuna shifter cable set.

Asking "Will I feel a difference" is silly because I know that has a lot to do with my own abilities and having been around the 41 long enough I'll get 25 different "it depends" answers.

So my real question is: Does upgrading cables typically reduce the amount of adjustment needed over your run of the mill stock cables? Do they typically have less stretch? Given that my cables only have 1000 miles on them, is it even worth the money right now?

Don't bother.

Not a bad idea to change cables yearly, so once the bike is a year old you could get the fancy ones. Even then, if your set-up is done correctly I suspect you won't see any significant difference.

I haven't found a set that didn't work great as long as there was nothing hindering the nearly free movement of the cable inside the housing or the tubing of the frame. More often than not, "bad" cables or housing can be attributed to something causing unnecessary friction. Something that can usually be easily rectified.

Only you can judge what is worth your money. Replace the cables as necessary with cables you think are nice. As has been written above, cables, given the proper maintenance are mostly trouble free.

Personally, I like Yokozuna and Alligator Ilink cables.

I really have never used expensive cables like Yokozuna etc, but I just replaced the cables on my road bike with Dura-Ace cables, and the shifting is just incredible as compared to where I was at with my two year old cable set.

And I was able to buy the shift and brake cables separately for cheap on e-Bay.

Just replace you cables when the current ones wear out. You won't notice much of an improvement, and I'm actually thinking of moving away from Yokozuna.

I just picked up yokozuna cables. I really like them, but I also got them at a team discount instead of msrp. They're pretty good, and I would say you can feel a difference, however it's not necessarily a performance difference, just a slightly different feel. They seem to eliminate a lot of the friction between contact point (lever/shifter) and it's end mechanism (brake/derailleur). Brakes have a little more feedback to the levers and ever so slightly decreased effort when applying the brakes. Shifting is a little better, too - rear derailleur is a little stiffer (but due to the mechanism/amount of throw required, it's not that pronounced). However, front derailleur feel is way better. Shifting is noticeably stiffer and more responsive, and not "mushy" at all. Shifting under load is noticeably improved.

Full disclosure, I also had replaced the stock cabling as well, which also eliminated some extra cable length around the cockpit (I dropped the stem several spacers from stock configuation), so that also may have eliminated some play from the stock setup. That being said, cable housing is VERY stiff, and I'm not doubting the "compressionless housing" claim they make.

All in all, they're nice, maybe even a slight luxury. I'd say, if you're in the market for new cables, definitely give them a try. They're not that much more expensive, and for something you only upgrade/replace once a season, it's not entirely cost inefficient to give them a try. If you have working cables currently, use em till you need new ones, and then give nice ones a try.

EDIT: oh, I also have Rival, if that makes a difference.

My cables are too new (2 weeks old now, I think), so I can't comment on long stretch/break in period, but if you remind me, I can let you know as time goes on. However, in the short time I've been riding them, I can say that I think they've needed less adjustment than stock ones. After my first weekend of racing (immediately after I got the bike), I needed to tweak the barrel adjusters on the stock cables (particularly FD). Now, after two weeks of training with the Yokozunas, I still haven't had to touch the barrel adjusters.

In my experience, any decent cables (housings are the most critical part) work very well when carefully installed. Higher end ones may go longer before needing to be replaced. I'll usually replace the rear DR loop a couple of times before replacing the rest.