I don't mask when contact printing, that'd be the purpose of the mat. On the second point, should the mat show the irregular black border and the film margins, I'm in two minds about it. On the one hand, why shouldn't something of the process be visible? On the other hand, it's kind of an in-joke, isn't it. Lots of civilians will think: "He didn't clean up the borders well." Best to let the process show up only in that way that it enhances the image, therefore mat cutout at the intended image edges.
Best regards, Christoph