As an atheist, I think I might have some tips for you:

If you think it would create a problem I would just avoid making a 'declaration'. With very religious people, declaring that you're atheist or agnostic usually is accepted worse than if you were to come out of the closet declaring you're gay. Despite the outcry of the gay community, atheists are actually less trusted according to polls.

If they're liberal Catholics, then you probably really wouldn't have that much a problem with it. But, I don't know your parents, so how can I know?

My parents are pretty liberal (maybe except for my father at many times), and they're Lutheran. I'm atheist, but I never got them in an awkward family meeting to 'declare myself'; I didn't see the point. Over time, after many philosophical or political discussions with my family, and after an obviously long period of not going to church, I think they just got it that I was atheist. Likewise, if you just let it settle in, they would be much more inclined to think that your decision to be atheist or agnostic is more grounded in your own personal decision and opinion, rather than a fleeting emotion or apathy.

I hope that helped you some.