Myself, I just use my actual opinions -- I'm so bad at lying convincingly, and even if I was better, I have better things to spend my cognitive faculties on than keeping track of which people I tell which lies to.

OTOH, that's a very bad idea with people who don't already know you. (In that case, I play it safe, e.g. in the past couple years I only ever mentioned my beliefs in a God or the lack thereof to people I had already known for months, or whose ideas on the matter I already knew.)