I suppose the answer to your first question is - it depends. It would depend on the purpose of your story and its intended audience - but having said that, there are ways of showing the attitudes of an age without it being the focus of the story, and without necessarily dwelling on them. And if those attitudes were something inescapable and universally present, then I think you'd have to show them and deal with them, regardless of the story's purpose. So if you want to write a light fluffy romance perhaps it's best not to set it between two slaves in the deep south during the Civil War unless you want to tackle some seriously unfluffy issues.