Resigned to replace chain AND cassette
 

Should I just wait until I get skipping? My chain is stretched just under 1/8" over 12". I can see gaps around the large chainring at the top and bottom. I've got 3000 miles on the drivetrain. I ride in the hills and have shifted under a load quite a bit. Should I be worried that I'll wear out my chainrings, or can I just keep going until I get skipping (or a broken chain)? Thanks!
Alan

I would replace the chain now , if not you will wear out the gear(s) on the freehub or freewheel which ever you have . Chainrings will last thou 3 or more chains if you take care of it.

You can try replacing just your chain but at 1/8" stretch it will likely skip on the cassette, which will then also need replacement. You might get away with not needing chainring replacement, or you might not, you will know by whether it works OK or not.

Don't wait so long next time, replace the chain at 1/16" stretch to avoid needing cassette and/or chainring replacement. http://sheldonbrown.com/chains.html#stretch

What I do in situations like this is buy a chain, and give it a shot, but save the old chain.

If the new one runs OK, you dodged a bullet. If it skips, you have choices. First and most obvious is but the cassette. Seond, which only works if it's very borderline and you can live with occasional skipping is to run the new chain on the old cassette. They often grow into each other and the skipping resolves, but the process can be slow and you have to live with the skipping for a while. Or you can switch back to the old chain and run it to the end, and replace both chain and cassette later. Sometimes this is hard on chainrings, but often not so. The key seems to be the size of the rings, larger rings are more forgiving than smaller ones, with rings of 36-4t in between.

There's lots of debate about the smartest, most miles for your dollar approach to drive train maintenance. Some replace chains early (1/2% or 1/16"/12") to preserve the cassette and can generally get 3 chains before needing to replace the cassette anyway. Others opt for the run it til it dies approach, and often get as much mileage out of a single chain then early replacers get out of 3. I rotate multiple chains running the entire set until they die, which can take a very long time.

No method is smartest for everybody, and it really depends on the relative cost of chains and cassettes, and whether you tend to wear out only one or two sprockets, or use and wear 3-5 fairly evenly.