The bible does say that foolish talking or jesting is not convenient in Ephesians 5:4. And in Ephesians 4:15 we find explicit instructions how to convey the message: "speaking the truth in love...". And as far as witnessing goes, Isa. 50:4 states that God had to teach Isaiah, "how to speak a word in season to him that is weary...". Again in Isa. chapter six we find Isaiah crying of his uselessness due to his unclean tongue and lips, to which God had to purify that member with a hot coal from off the altar before He could use and send Isaiah to witness to others. With this said, I believe we find an answer concerning the idea of including sarcasm in our witnessing.