It depends on the situation. I usually try to be truthful, but I don't see an issue with the occasional lie. I'll be truthful with the police, significant others, really close friends, and my work (if I screwed something up at work, I'll own up to it. If someone else screwed up, I'll just say I'm not sure what happened). I am almost always truthful with my family, but there are exceptions (this used to be more of an issue when I was younger and smoking pot and drinking alcohol, but as I do neither of those things any more I haven't had to lie to family in a while).
In regards to friends, I try not to lie but I don't see a problem with lying to them. There are some things that don't necessarily concern them and I won't tell them or just omit certain parts of the story if they ask, or if I know that telling them the truth is going to hurt their feelings I'll lie. I do try and be truthful if they need advice though. I used to be brutally honest and it cost me some friends, so I've learned when and where I should lie instead of telling the truth.
edit: It's much easier for me to tell the truth on the internet since I usually don't know any of the people I'm talking to and telling the truth won't have repercussions the way it would in real life.