If you want to be able to run it inside cmd.exe or batch files you need to add the directory the .exe is in to the %path% variable (System or User)
If you want to be able to run it in the Run dialog (Win+R) or any application that calls ShellExecute, adding your exe to the app paths key is enough (This is less error prone during install/uninstall and also does not clutter up the path variable)