It's even worse than that - if you tag off at the wrong station, your monthly pass gets locked in at that other station. You can't change it. I had a friend who traveled TO work on a shorter route than when he traveled back FROM work - so on the 1st of every month he had to drive to a farther station from his house to ride the train in, lest his card lock him in to a shorter route than he needed.
Monthly passes are also zone-based, so on the off chance that you are making a cash-based trip outside of those zones, you still need to tag in and out for that trip.
It's been a while, but I seem to remember, if you have a monthly, you only need to tag in and out on the first trip of the month to activate it (no refunds after that).