We do not need to forget about it but I think we should put it into perspective. While we recognize slavery as a great wrong today, it was legal and acceptable back then. Jefferson broke no laws, and, according to his associates, his slaves were treated more like house members.
People have to be judged by the era in which they live. Two hundred years ago, virtually ever man in America would laugh at the idea of women voting. They simply didn't understand that women are often better decision-makers than men. That doesn't make them bad, it just makes them products of their time.
Leonardo Da Vinci said : 'The time will come when men such as I will look upon the murder of animals as they now look on the murder of men.' That's very likely true, but it would be wrong for people 200 years from now to look back on the meat-eaters of today and judge them by that not-yet-evolved habit, because, in today's world, it's not considered wrong. Well, not by all, but you won't catch me eating the flesh of other creatures.
Same with our Founders. We have to judge them by the era in which they lived.