A fairly depressive thought but quite an interesting concept. What if the words you said to a person would remain as the last words you ever say to that person? You can never know if you will be able to see someone again or talk to them.
How about ending every conversation with words that are so important that you could have no remorse if you never see the other participant of the conversation again? Is that even possible? Words that matter are so rare and occasional. 'I love yous' and the likes only matter when said to right person at a right time. Ending a conversation with an insult though, that's bad. That's something I'd get remorseful over.
I suppose we just have to choose the words that come up in that moment, regardless what the outcome might be. Honesty does wonders.