For most people, it's hard work to lie and it often isn't very convincing. It makes me feel uncomfortable and I would rather not lie. But I'm sure there are occasions when it is justified, as well as others when it probably isn't.
If I was out shopping with someone and they asked my opinion of an item of clothing, which I thought was unflattering, I would say something like "I don't think that style/colour/length, etc., etc. really does you justice." However, if someone was just about to go out in the new dress they'd bought and asked my opinion, I would probably say it looked fine because I wouldn't want to spoil their evening.
Things get more difficult with serious matters. If, for instance, you know a friend's partner is having an affair, should you, if asked, confirm your friend's fears? Or, even if your friend just voices concerns about his/her relationship and asks for your opinion, should you lie by omission? Of course, one might feel it better to be completely above board but I'm not sure it's always the best thing to do. It would depend on the circumstances.
I generally think it's better not to lie to children, even to protect them. A neighbour's much-loved dog died two Christmases ago and she told her grandchildren that he'd gone to help Father Christmas deliver presents. When Easter came, she said he'd gone to help sort out all the Easter eggs - and so it continues. I don't know if these children have caught on yet but it seems unwise to shield children from real life in this way and may make them distrustful in future.
Re Father Christmas - I was very upset to find out it was my Dad who had delivered my presents. I and three other innocents/fools had argued vehemently with the rest of the class that of course there was a Father Christmas, so, apart from anything else, I felt really stupid.