Is it safe to to upgrade OS X 10.7 to 10.8 on a productive system?
Most of the time the OS X upgrade routines work quite reliably and in this case there shouldn't be any deal-breakers like Rosetta back in the days. However, it is impossible to tell for sure. What I usually do, is to clone my system to either a second partition in the internal drive or to an external (USB) drive (using a tool like SuperDuper! or Carbon Copy Cloner).

After booting the clone (press alt in the boot screen) you can safely upgrade this partition to a newer version of OS X and play with it for a while. Once you are sure everything works, you can either copy it back to your main partition or upgrade that one too.

Nevertheless, I recommend to have another (e.g. TimeMachine) backup available, in case anything goes wrong.