Every word counts. Every word we speak is heard by God. Every word matters. It's no use trying to use the excuse, "I didn't know what I was saying" or "I was just so upset that I couldn't control my mouth." Now, I know I've used those excuses myself and sometimes I try to think "it doesn't really matter because God knows my heart." I will say this, God is certainly understanding of our weaknesses, but those "idle words" were still wrong, sin even, and Jesus had to die for those as well.
I've used Psalm 19:14 before as a Daily Verse, "Let the words of my mouth and the meditation of my heart be acceptable to You, O Lord my Strength and my Redeemer." That should be our prayer every day because our words show what is in our hearts and people listen. We need to speak "wonderful words of life" which may "impart grace to the hearers" Ephesians 4:29b. Our words can heal, Proverbs 15:23, but our words can also hurt, Ephesians 4:29a, "Let no corrupt word proceed out of your mouth..."
Note Proverbs 10:19, "In the multitude of words sin is not lacking, but he who restrains his lips is wise." As James told us in 1:19, "be slow to speak..." Take care of what you say. People hear and God listens, too!