The Fare Inspectors will fine you if you haven't bothered to swipe your card, even if you have a monthly pass. The card stores a record of the last $WAY_TOO_MANY interactions with a Clipper Card reader, so it's trivial for them to check. Those guys have no sense of humor. :(

I'm a huge fan of Caltrain's policy: Monthly pass holders only have to interact with a reader twice at the beginning of the month [0]; once when you board and once when you disembark. For the rest of the month the conductors use their brains to remember the current month, and compare that against the end date on your monthly pass.

[0] And -realistically- you don't even have to do that. I went a couple of months without interacting with the Caltrain readers; the conductors never said a thing.