No, an anti-possession mark doesn't have to be over the heart. In season 8, Kevin and his ninja mom, Linda, got them on their arms - but if you remember, Linda's was burned off so that Crowley could possess her.
To make it virtually impossible for anyone to possess the Winchesters, Cass put Sam and Dean's anti-possession marks, visibly, over their hearts, but there was mention that he had marked them deeper, at the very least the markings went through to their rib cages, but it makes sense that the marks actually went all the way through to their hearts. Besides the symbolism (and convenience since the scene would have been very different if he'd reached down and grabbed them by the ankles instead of placing a hand on each other their chests), the basic anatomy would just be too much to burn off or out if the marks were placed that deep. In other words: Burnt/removed ribs + burnt/removed heart = No decent meat-suit to walk around in, so no reason to even bother trying.