It depends how comformtable you are in the language. I won't go to some old lady in Russia and say "Твою м**!", because she will die of a heart attack. If people think you are a foreigner they are more forgiving with swear words, but if they think that you speak well enough russian to converse, they might not be so forgiving...
I never would go to some old lady and say "Тв*ю мать". Not because she will die of a heart attack. Because I'll feel very 'caitiff' if I do so. It depends what people you are talking with. If you are talking with a rude man he's likely to say lots of swear words that some natives never heard.