Does being under grace mean Christians can sin as they please?
No. Being under grace does not mean Christians may sin as they please; that is not grace. Scripture answers such reasoning with 'God forbid.' God's mind on sin has not changed, and love, the fulfilling of the Law, moves believers away from adultery, killing, false witness, and coveting.
Les explains that grace is not a license to do as we please or to sin as we please; he says, 'No, that's not Grace.' He points to the next verse's question—Shall we sin because we are not under Law but under Grace?—and the answer is 'God forbid,' meaning don't think such a thing. Believers are still under God's providence. Although they are not under the Law as such, with its stated penalties, God's mind on sin has not changed. Les says God hasn't changed, and the things He told Israel in Leviticus are just as abominable in His eyes now. Under grace, love is the dominant force in the believer's life; love fulfills the Law, so when love operates, one does not commit adultery, kill, bear false witness, or covet, and any other commandment is summed up in loving one's neighbor as oneself. Those statements come from the Ten Commandments, and Paul repeats nine of them, omitting the Sabbath.
What then? shall we sin, because we are not under the law, but under grace? God forbid.