Yes, if you are not going to use FMP12 then it is safe to uninstall. It is not required to be on your computer to install the FMP13 upgrade. It will ask for FMP12 key if it is not found on the computer.
But it can be useful to keep older installs around if you are a consultant and work with multiple clients. Some may be using an older version and not be willing to upgrade. Having the older version let's you double check that your solution will work OK for them in the older version of the DB.