chule,
the park break wire is an interlock to ensure that the park brake is on when the timer keeps the engine running, i.e. if the park brake isn't on, as soon as you turn the key off, the engine will stop and not continue to run via the timer. I tied mine to ground as couldn't be bothered getting to the park break wire. so its a question of risk, if you are the kind of person to forget to put the park brake on when leaving the car you should connect it, otherwise, if you are already in the habbit of putting the handbrake on (kind of like a mate of mine when we were bogged in sand up to the axles, still put the hand brake on), its not really necessary.