The New Testament speaks against careless words (Matt. 12:34-37), lies (Eph. 4:25), unwholesome talk (Eph. 4:29), blasphemy (1 Tim. 1:20, 2 Pet. 2:12), coarse joking (Eph. 5:3), obscenity (Eph. 5:3), foolish talk (Eph. 5:3), insulting terms (Matt. 5:22), slander (Col. 3:8), filthy language (Col. 3:8), boastful words (2 Pet. 2:18), and cursing (James 3:9-10, Rom. 12:14) to name a few. It would be hard to read the New Testament honestly and come away with the idea that it isn't very important what we do and don't say, which words we choose to use and which we choose to omit. Few of us fail by being too careful of what we will say, most of us have areas in which we are far too careless about the words we will use, and the way we will use words.

Matt 12:34-35
34You brood of vipers, how can you who are evil say anything good? For out of the overflow of the heart the mouth speaks. 35The good man brings good things out of the good stored up in him, and the evil man brings evil things out of the evil stored up in him. NIV