This question popped up because just yesterday a foreigner used one of our slangs and it sounded really off and awkward. I know he was trying to blend in and sound cool but it just didn't work out. This is not the only scenario I've seen, I've seen countless of this stuff happen and most times,it doesn't go as planned in your head.
As for me,it's a yes and no in my opinion. I’d say that knowing slang is integral to comprehension but using it totally depends on the circumstances.Slang in French can be hard to keep up with as is true of all languages but the stuff that is here to stay will keep you abreast of what is being talked about. In terms of using slang then I’d say use soft slang pretty carelessly. Use taboo slang with people you know won’t mind or if you’re too drunk to care. Use the taboo words with extreme caution – only use during a fight with your French lover or if someone half-inches (pinches/takes) your drink.
The long and short of it is, If you don’t want to cause offence then be sure of the meaning, connotations and choose your audience wisely.