I've thought about lying a fair bit too. I think you're right that it is a complex issue and handling it too simplistically does not serve our kids. Adults say, "Always tell the truth," but then we don't. We lie about how much we like a present, we like about being awakened by the phone, we lie about all sorts of stuff - tons of times about totally insignificant things. 

I have talked about with my son about people earning our truthfulness. If it is someone we know, love, and trust then they are a different sort of person than someone who is an authority, a competitor or a scoundrel. It also addresses why I want to get the truth but don't expect him to always tell it to others.

It took me some time to bring some understanding to constantly playing by a different set of rules than others did. Now, if people are trying to trick me, I don't feel too badly about trying to trick them. Perhaps it is moral relativism but especially regarding my kids, I don't want them to ever feel that shocking vulnerability that comes from betrayal, especially if s/he knew the betrayal was likely to happen.

It reminds me of that opaque passage where Jesus says, "Be shrewd as serpents and gentle as doves." Mt. 10:16 Taking anything that small from any text it is open to misuse but I think moral people are often constrained by their simplicity to being consumed by the immoral people. We needn't be.