This will be the first major OS update since the introduction of iCloud. The beauty of iCloud is that it backs up most of your critical data like your contacts, SMS, notes, etc. What you might loose is app data so yes there is a possibility you may loose your progress in Temple Run. But my experience usually saves even app data.
Since iCloud is also Apple server side I recommend running an iCloud back-up a day or two before you update. And as nutmac said, back-up to your computer as well before you update.
The big problem with people losing stuff on their phone when iOS5 came out was the phone would get hung up during the update process, due to the large surge on the servers from users trying to update, and have to be put into DFU mode, I myself had to do this. And then it would need to be restored from a previous back-up, I had to do this as well. Since a large majority of iPhone users have never even connected their device to their computer or sadly aren't even aware of doing that, they lost their data. So, to insure you get all your critical data from your phone back if this scenario happens, make sure your device is set to back up things like notes, contacts, apps, etc. on your computer through iTunes when the device is connected.
If you follow these steps and take the proper proactive steps to save your data then you will be ok in an instance where the phone gets hung up and must be restored during the update process.
