The passage in Romans 6:7 is an arresting text.
"For he who has died has been acquitted from [his] sin" (NWT).
"for he who has paid the penalty of death stands absolved from his sin"
"For he that is dead is freed from sin" (Webster's).
"for a dead man can safely be said to be immune to the power of sin" (Phillips).
Certain scholars and theologians understand the language of justification/being declared righteous to signify that God has actually made a Christian righteous, whereas most would probably say (including Jehovah's Witnesses) that worshipers of Jehovah are imputed righteous upon the basis of Christ's sacrifice (not actually made righteous). To quote Romans 4:4-8:
"Now to the man that works the pay is counted, not as an undeserved kindness, but as a debt. On the other hand, to the man that does not work but puts faith in him who declares the ungodly one righteous, his faith is counted as righteousness. Just as David also speaks of the happiness of the man to whom God counts righteousness apart from works: 'Happy are those whose lawless deeds have been pardoned and whose sins have been covered; happy is the man whose sin Jehovah will by no means take into account'" (NWT).
Paul apparently teaches that Christians are accounted righteous--they are reckoned as being righteous in this life (not actually made righteous by the divine act of justification). Furthermore, in order to maintain a right standing in the sight of God, one must produce works that show evidence of his/her faith.
Romans 6:7 also indicates that one who has already been acquitted of his/her sin has undergone a symbolic death whereby imputation of guiltlessness has been effected (brought about). See Hebrews 10:14.
These words from Acts 13:38-39 might bear on the matter at hand:
"Let it therefore be known to YOU, brothers, that through this One a forgiveness of sins is being published to YOU; and that from all the things from which YOU could not be declared guiltless by means of the law of Moses, everyone who believes is declared guiltless by means of this One" (NWT).
"As a man that is dead is acquitted and released from bondage among men, so a man that has died to sin is acquitted from the guilt of sin and released from its bondage" (Henry Alford on Romans 6:7).