Most thieves won't recognize a Moulton for what it is, and would probably pass on a well-locked "cheezy folder" for the BSO "mtn bike" sitting next to it with a flimsy cable lock.
That said, I'd lock a Moulton with top-line locks, either U- or one of the hardened chain styles, and never an all-day affair, only for running in and out... and for which errand duties I'd probably pick a different bike than a Moulton, anyway.