Lies lies lies

Is there ever an instance where lying to "keep the peace" is acceptable? Like. "I lied because you would get mad and I didn't want to start a fight" ... Does it matter what the lie was about? Say, who exactly was at a place you went, porn on your phone when you said there was none, money in the bank?

And further to that, do people who lie constantly really think it will never be found out?