When can the "F" word be acceptable to you in a conversation?

When children are around, or when in public and people who you don't know are in earshot. I find this repulsive. When I used to shop at Walmart, inbetween the chatter in Spanish, I'd hear people talking on the phone or to each other and using the word like it was nothing. Children were close by, 5 and 6 year olds, people who might find it offensive are close by, and it's just not acceptable. Use it when your conversation is private or when you're with a group of people and the language bounaries have been established, or at least familiar enough so if someone says, "Hey, I don't like that word" it's respected.