I think that lies of omission, when they don't directly or intentionally cause pain to you or others, are just fine. They are lies and I don't justify lying at all but I believe it doesn't hurt to keep some things secret. And in many cases, secrets are equivalent to lies of omission.
I suppose starting out explaining that I have created a rule that you should tell lies under specific circumstances demonstrates that I actually have an issue with this rule. I do. My issue isn't the lies of omission as much as it is my inability to keep secrets.
If a friend tells me something and tells me specifically not to tell anyone, my first decision is to forget about it. That's the best way not to tell anyone. And often it works well for me. But sometimes those secrets are juicy or similar to someone else's issues or are something I want to help them through. And in those cases I do tell their secrets – to people who are highly unlikely to cross their paths and I change names, dates, and places to protect the secret. Since I know that telling someone is not keeping the secret, I feel guilty and that I betray my friends and family even if my intentions are good.
So, more to the point, keep a secret garden. Don't tell everyone everything. Keep some secrets.
Use a journal to write down your secret events, secrets you've learned about yourself, and secret feelings you have toward others. I first began using my journal as a secret garden during a period of time where I was always angry – wearing a uniform for battle (everything I owned was black and olive green like fatigues), refusing to wear my hair in conventional styles (wetting it and letting it air dry while driving with the windows down), and blasted heavy and rebellious rock music every chance I got. I was also blowing up on people, telling them exactly how I felt and what I thought at the exact moment. Having heated discussions does not nearly adequately describe the situations. Once that began to seep into my everyday professional life, because I was keeping those issues with close friends and families – attacking loved ones and rather than strangers, I realized if I wanted to keep jobs and provide superior customer service, I needed to get my anger under control. Instead of outbursts, I walked away from situations that made me angry, and wrote a letter in my journal to the "bitch" or "asshole" I had recently come in contact with (that's how angry my letters were – I resorted to name calling). After writing down every angry feeling I had, I realized I had a part in our disagreements and could only change me. It helped significantly.
Keeping secrets to yourself, forefront in your mind, gives you intrigue and happiness inside. I recall one day leaving work for a "lunch date" and when I returned, only vaguely answered the questions my co-workers had about how my "date" went. As I thought of the parts of the story I didn't want to tell, I became giddy. My "lunch date" was myself and I went home and had wine and cheese before returning to work. For me, a half a glass of wine in the middle of the day and then returning to work was "scandalous" and something I definitely didn't want getting around the office and especially not to my boss. But no one knew and no one needed to know and not only did I feel giddy from the secret, giddy from the special lunch I decided to have just for me, and giddy from my "scandalous" behavior, but my co-workers created this mysterious and adventurous ideal version of me that actually improved our working relationships without my having to tell everything about myself (or make up stories of intrigue). Keep some things to yourself.
And lastly, TMI (too much information) is unattractive. No one should be subjected to that except the professionals and close family who have to help you through the situation.