Personally, I don't run omnis (bc I don't have any!) but omnis are generally used closer to the sound source and should be spaced roughly 18"-36" apart from one another, as the stereo image is based on subtle time differences to the mics. I would stay away from omnis in chatty crowd situations or far from the sound source, but under ideal conditions omnis can sound amazaing!
Any omni users have more detailed instructions?