If using old school soft lead rifled slugs I don't see how it could possibly hurt anything. They are designed to expand at the base to fit the barrel and are perfectly safe in even full choked barrels. You will probably see better accuracy with the more open choke, but it can't hurt anything.
The newer sabot slugs might be a different story. These are designed to be fired in fully rifled barrels and might cause problem in traditional barrels choked wrong.