First of all, you should know that the data in your iPhone can only be seen but you have no access to them.
Many iPhone users are very care about the security of their iPhone data, so do I. I used to connect my iPhone to computer and synced the data to computer at the same time. Once I want to recover my data of iPhone, I will use the iPhone backup extractor to export my data to iPhone again, it does a lot of good to my iPhone data.
You don't lose anything if you back up your phone before upgrading. I made 3 backups; one to iCloud and two to my computer. The way to keep multiple backups is to go to iTunes Preferences -> Devices, right click on the most recent backup, and choose "Archive". The next backup will be a full backup, and you will have both available.
If you back up over the air you also will not lose anything unless something goes wrong in the upgrade process, but then you have your backups to restore from.