Atheists are familiar with the Grandma Gambit — it's a common tactic used to shut us up. We're told to keep quiet because our dear sweet devout Grandma couldn't possibly deal with the news; it would break her heart and you wouldn't want to do that, would you? What kind of callous rat would hurt a gentle little old lady!
It's a rather patronizing suggestion that belittles Grandmas everywhere. Both of my grandmothers were feisty types who would have relished a good argument (and one of my grandmothers, who died when I was 12, would probably have just said, "good for you"). Go ahead, break the news to Grandma — it's much more respectful than treating her like a delicate flower that would wilt at the thought of you not going to church.
Here's a more realistic reaction from a Christian grandmother who hears that you've left the faith: an argument, in the form of a 33 page handwritten letter which is almost entirely a creationist screed. It's interesting, too, because I see this a lot, that nowadays the response to apostasy is often built around arguments against evolution. There is an expectation that faith is not enough, and that calling the faithful back to the fold is a matter of reasoned argument with 'science' on their side. Unfortunately for them, they don't have any science at all, and Grandma's letter is a series of creationist canards, from the "just a theory" error to the absence of transitional fossils, all wrapped up with bible verses.
So Grandma wants to talk; what does the grandson do? He writes back with a 17 page letter, neatly typed, with charts and figures! Bravo! This is how loving families should deal with faith, by simply caring enough to wrestle with the ideas between them.