My wife is a research historian, and she would like to remind us that this is not an "either-or" situation. When learning about the past, we must consider context of historical attitudes and actions to get a clear idea of intent and social expectation. But we must also relate them to their context today, and frame them with the ethos and social expectations that we are living in. It's vital that we do both. We must measure past events by their past context, and measure them by their current context. Failure to do either is a failure to learn, and a missed opportunity for progress.
She is glad that you mentioned Thomas Jefferson, because that is a good case-in-point. Yes, he was a slave owner. But slavery was not "acceptable" back then. In fact, it was considered morally abhorrent in most of the Christian world at the time. The abolitionist movement started in the 1600s when the Roman Catholic Church condemned it, continued to 1732 when James Oglethorpe publicly argued against slavery in his proposal to found the Georgia colony, and gained strength in 1777 the brand-new State of Virginia abolished slavery. Slavery was a major point of contention among the nation's Founding Fathers even before the Declaration of Independence was signed, but it seems that those facts are never part of the discussion today. We are too quick to just tell ourselves "well it was okay back then, but times were different" and not measure that difference (and especially the similarities) between Then and Now.
Slavery was not "okay", not even back then, unless you were profiting from it. It just so happened that slavery profiteers controlled most of the land and wealth (and therefore, most of the political power) in America for more than a century. That is a very important thing to remember, especially in the modern era. The complete picture is so much more applicable in both time frames.
So when you find yourself saying "well that author might have been a complete jerk, but everyone was a jerk back then so it's okay," stop and ask yourself...who said it was okay? Was truly everyone a jerk back then? and if so, was everyone truly okay with it? What changed, and why, and how?
She also recommends we read more history books and fewer works of fiction, but she gives that advice to everyone.