In Season 1, almost anytime Livia mentions Johnny Boy, whether it be to Tony, to Carmela, or anyone else, she refers to him as a "saint", she says Junior couldn't shine Johnny's shoes; That Johnny would never have let her rot in a nursing home....Yet in the flashbacks to the '60s and '70s when he was alive, it's made clear he was a pretty terrible husband to her, and she seemed well aware of his misdeeds (for example his infidelities etc).....

Why do you think she referred to him as a "saint" and whatnot after he died?

Im not so sure Livia considered Johnny a saint period. I think she just said it to make Tony look like shit. Also, it's pretty clear that Livia isn't playing with a full deck if you know what I mean. But I would say its just another way of her making Tony look like a bad husband or piece of shit to Carmella.

I think Livia was just so dysfunctional and downright mean,that she enjoyed finding the soft spots in people and saying the things that she knew would hurt them the most. She really had a gift (as twisted as it was) for manipulation.