I say Hell yes they should let people know about this. It just goes to show that times change.
If you try to make Washington, Jefferson (or Jeff Davis and Lincoln for that matter) into something they are not, or something more than they are, you are guilty of a lie or a sin of omission.
Obviously, slavery is not the most important issue on the site of the signing of the Declaration of Independence. Buy when you think that less than 90 years later the Southern States would try to dissolve the Union set there over that very issue, it becomes an interesting footnote.