I will only monitor them on the treadmill, bike, stair climber or any other cardio equipment if it’s part of their workout and it’s done during the hour they are paying for. If they do the treadmill any other time, then I might give them some directions and/or tips (if needed), otherwise they are on their own.
I would definitely recommend monitoring their activity as you should be concerned with every aspect of your clients’ training. I also take this opportunity to assess my clients response to our last session and use their feedback to fine tune the current session. You can also check their level of exertion while doing this cardio to make sure they are reaching the zone they need to be in. You can never get too much feedback from your client at any time.