There are some seatbelts that do not lock adequately and would require a heavy-duty locking clip to take up all the slack. The ClickTight would presumably not be compatible with those. Since you have a 2005, it should technically meet the lockability requirement as it was mandated in the late 90s.
As for booster mode, it isn't necessary to lock a shoulder belt retractor, but it is usually fine to do so. In some cases, locking it can prevent the child from adding slack during a trip.
As for booster mode, it isn't necessary to lock a shoulder belt retractor, but it is usually fine to do so. In some cases, locking it can prevent the child from adding slack during a trip.