People adding you to circles only means they will see what you write PUBLICLY. If you don't add them back, you won't see what they post. And if you only post to your circles (and not public), they'll never see a thing you post.
But it really doesn't matter if they do or not. It's kinda like if Mary Sue Smith in Foley says you're her best friend. You don't know her. You don't know she's calling you her friend. She has never spoken to you.