I always renew for the reasons irev210 mentions above. I will sometimes hold a renewal for a certain device, but the only reason not to renew is flexibility to leave. If you plan on staying, you might as well renew and get the subsidy on the next device. Losing approx $20 per month in value per line on your bill seems pointless.
In my case, I have 4 lines. That's $80/month in value total. Cant let that get by. Right now, 2 of my lines are out of contract. However, I just preordered the GS3 today on contract. I will be left with just one line off contract...ready for a new device should something tickle my fancy.
Robert via Kindle Fire using Forum Runner