You should always be honest
It's in your own best interest to never lie. Honesty can be extremely uncomfortable in the moment. Most of the white lies we tell -- which we view as best for everyone involved -- are actually the very things that allow us to get stuck in bad and awkward situations and relationships.
If you are completely honest with the people around you, you have to tell the person trying to hang out with you too much that you actually just aren't interested instead of pretending you're busy. For this one awkward moment -- one moment of facing the discomfort head on -- you avoid prolonging the same uncomfortable situation. You wouldn't have to ask, "why can't this person tell that I don't like them?", and they wouldn't have to wonder whether you do, if you just told them the truth. This fear of telling the truth when it's uncomfortable gives birth to much more discomfort than necessary for both people.
It's also good to tell the truth because then everyone around can actually trust your word. It may be worth slightly offending someone when accepting a gift so that they know how truly you love the next thing they get you. (Although I think this is probably the most harmless white lie, it can still cause unnecessary future discomfort: "Why haven't you worn the sweater I got you for Christmas yet?")
This isn't to say that you shouldn't lie in self defense ("where are you hiding your children?") or that you always have to tell the whole truth and say every single thing you think. But not lying almost always seems worth it. When you lie you're trading short-term comfort for long-term confusion and discomfort.
TLDR:
It's in your own best interest to never lie. When you lie you're trading short-term comfort for long-term confusion and discomfort. In what situations do you actually want to be on the other side of a white lie? In what situations would you prefer someone pretend to want to hang out with you and cancel, rather than them just telling you they don't like you?
References:
The Almanack of Naval Ravikant
I never want to be in an environment or around people where I have to watch what I say.
Lying by Sam Harris