Just put a new SRAM chain on my commuter bike, which has a 9-speed 11-34 Shimano cassette in the rear. Had a terrible commute to work -- chain won't stay in the 3 largest cogs without skipping around.

I believe this means I need a new cassette -- the existing one has about 4,500 to 5,000 miles, a lot in the winter grit. Any reason not to get a new one -- I have a hard time judging "tooth" wear on the cogs.

Probably.

And do it soon. Too much riding on a worn cog will ruin your new chain.

I agree that's the most likely answer. Before spending the money, however, I'd want to check the derailleur adjustment and hanger alignment.

Also, if you still have your old chain, measure to see if it's "stretched".

Thanks for the input.

Just to close the loop -- first, I did measure the chain. It was stretched, which was why I replaced it. Second, I discovered (to my embarrassment) that the really serious problem I was having was caused by the fact that I had improperly remounted the rear wheel -- the QR skewer was not tight enough, and the wheel was slipping under load. That's what was causing the slippage in the larger cogs.

Finally, I do think this cassette is worn, though -- I'm going to replace it.