Whether or not Apple will officially release iOS 5 at their special event tomorrow is still unclear, but in the event they do and you want to upgrade it's prudent to get everything backed up in anticipation. Why?
In general, it's good practice to back up your iDevices before an update but iOS 5 also brings plenty of new wireless syncing options that could go horribly wrong. You can now sync with iTunes over Wi-Fi or to the cloud with iCloud. It's unlikely that anything will go wrong and there's certainly no need to panic, but this is new technology we're talking about and it never hurts to be prepared in case something does happen. If you're looking forward to upgrading ASAP, spend a little time tonight getting ready so you'll be able to recover swiftly in the event of a problem.