Actions speak louder than words. I don't expect non-believers to have the same morals, etc. as true believers. So it really doesn't accomplish much to be up in their face saying "You shouldn't do this, or that, yadda yadda." However, I'm not going to lie to them and tell them I approve of anything I don't. I've had people say "you're different" etc. etc. and that can open up the conversation about God. It's up to them whether they want to listen or consider it or not though. It's their choice.
One thing that bothers me I see and hear A LOT these days is the whole idea that we are a good witness only if we are everybody's friend and buddy, and everyone likes us, etc. etc. Love peace and chicken grease...whatever.. Of course we should love. We're called to it..
However, I realize not everyone is going to like me because of who I am in Christ. Can't be everybody's buddy, and some will even hate me. The world hated Jesus too. The creation certainly isn't greater than the creator.
“If the world hates you, keep in mind that it hated me first. " - Jesus, John 15:18
So love, but be realistic about it. That's how I see it. If someone says they don't want God, I'm going to still treat them nicely. If they say they don't want God or anything to do with me either, then see you later.
Of course when it comes to talking to people who claim to be believers already, that's totally different. They should be held to a higher standard. I'm not sure if that falls under "witnessing" though.