So do not let what you regard as good be spoken of as evil. (ESV)
To understand this verse we need to continue to look at the context. Paul is dealing with those who have embraced freedom in Christ and those who are not sure if that is a safe thing to do. All of this is because of the impact that the Law had on it's followers. Before Christ came into the picture the Jews rigidly attempted to live according to the Law.
We may wonder why anyone would want to continue pursuing the Law when it was no longer necessary. It would be easy to think that at the first sign of freedom in Christ, everyone would abandon their legalism and bask in their new found freedom. That is not what happened. There were some who embraced freedom in Christ. Others were so set in their ways they feared freedom.
For those who feared freedom they were concerned that if they acted in freedom they would appear to be very unspiritual. One of the by-products of following the Law was fixating on how you would appear to others. Do you remember how Jesus criticized the Pharisees for being so concerned with how they looked they had ignored the condition of their heart?
Paul tells the ones who were celebrating freedom in Christ to not let their freedom to be spoken of as evil. If they were unloving in flaunting their freedom the ones who were still struggling with freedom would view this flaunted freedom as an evil thing. Freedom in Christ was wonderful, but it could have been perceived as evil if it was not presented properly.
No matter what you do, consider your brother or sister in Christ first. You might be free to do something as a Christian, but your testimony for the Savior is more important.