I'm still very much a student of Objectivism, but I'll share my experience anyway...
My mom considered herself a Christian. She always wore a cross around her neck and kept a bible in the house and we 'said grace' before eating on holidays, but that was about the extent of it.
I became an atheist a few years ago, but I didn't talk about it much (like I didn't talk about my previous religious beliefs) and I don't think my mom realized I no longer believed in god.
She battled cancer last year before it took her life. Her belief in god seemed to give her comfort near the end and I think that knowing that I didn't believe would have really depressed her.
My thought is that, until I feel it's a sacrifice to keep my beliefs to myself, why should I share? Of course, keeping your beliefs to yourself, and lying are two different things... and I'm glad I wasn't put in the latter position with my mom.
Billy