It is not compulsory that you must tell them but when they ask and or when the need arises then you can preach the good news about Bitcoin to them. Many people are still working on the dark side of the world but telling them about Bitcoin if they are interested will bring them to the light of Bitcoin and it will shine in their lives. Be a good Ambassador in your locality and represent Bitcoin to the core.
I agree in principle, promoting Bitcoin is cool. But overall, I'd rather promote the P2P concept than Bitcoin itself. It keeps me from attracting attention, and those who understand the value of P2P systems will eventually find Bitcoin and understand it themselves if they bother to.
However, I think that in some cases, it's better not to promote than to promote in the wrong way. Stuffing people is counter-productive, and saying things that are wrong and misunderstood is a very bad publicity.
I was once in a hotel in Estonia, and a young Belgian was making pro-Bitcoin propaganda. I thought "Nice !". I pretended I didn't know anything about BTC, and I realized after 5 minutes that the guy knew almost nothing about it, and was talking nonsense. This is counter-productive, I don't think it does the community or Bitcoin any favors. I hope that, as time goes by and more resources in multiple languages become available online, some of the propagandists will improve.