I would have to agree with southerndreams - it really depends on what the 'omission' is.
Do I need to inform my husband and boyfriend of which one has a larger penis or can last longer? I don't see a benefit of giving either of them that information, unless either one wanted specifically to know.
I 'omit' certain details about my boyfriend because there are some people in our lives that are not comfortable with the idea of him or they just don't know yet. He is moving into our basement suite, but that's not something that is going to be known to everyone because quite honestly, we're not mentally prepared for the family backlash just yet. 'Omitting' this fact is, at least temporarily, better for all concerned. Is it 'lying'? Yeah, probably. And I have no problem with the consequences of that because it's being withheld for the sake of protecting myself and those I care about.
Many people keep their poly relationships secret from the world because living openly is not a practical option. Discrimination, problems at work, even losing custody of children can all be consequences without 'lies of omission'.
Can you tell us what it is that you're concerned about omitting? That might help you find some insight into your problem.
"There are worse crimes than burning books. One of them is not reading them." - Ray Bradbury