Did Old Testament sacrifices perfect the conscience of the worshiper?
No, the Old Testament sacrificial system did not perfect the worshiper's conscience. According to Hebrews 9:9, the gifts and sacrifices offered could not make the one who did the service perfect as pertaining to the conscience. The blood of animals could not take away sin; those sacrifices were a figure for the time then present.
Les explains that Hebrews 9 rehearses the Day of Atonement, when the High Priest alone entered the Holy of Holies behind the veil once every year, never without blood, which he offered for himself and for the people. The Holy Spirit signified that the way into the holiest of all was not yet made manifest while the first tabernacle was standing. The Old Testament economy, under Moses and the Law, was a figure, symbol, or illustration for the time then present. Its gifts and sacrifices could not make the worshiper perfect as pertaining to conscience. Les emphasizes that this whole sacrificial system in itself did not take away sin; the blood of animals could not take away sin. He is making this point in connection with baptism not being totally new to the Jew when John the Baptist came.
Which was a figure for the time then present, in which were offered both gifts and sacrifices, that could not make him that did the service perfect, as pertaining to the conscience;