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.