Important: macOS High Sierra
If you plan on upgrading to macOS High Sierra, it's very important that you have a proper Time Machine backup, and preferably an additional backup using a tool such as Carbon Copy Cloner. If your machine has a built-in SSD (and most current Macs do), then the upgrade will automatically convert it to the new APFS file format. As a result, if it turns out that any of your software doesn't work with the new OS then it is very difficult to go back to the earlier version. To complicate things, Apple has removed access to the previous os installer, so it's not as easy as wiping the drive and downloading the installer again.