There is always the possibility that some of your apps might not be compatible with Mavericks or that some unexpected problem might occur during the update process. Without knowing anything about the apps or third party add-ons you have installed on your system it is impossible to say anything more specific than that.
You should always have a backup of at least your most important files, if not everything on your startup drive. This is never less important than when upgrading the OS. The best & safest approach is to clone the drive to an external one using something like SuperDuper! or Carbon Copy Cloner & upgrading either the clone or the original but not both. Test your apps while started up from the upgraded OS & if all seems OK, you can begin using it for daily use.