One thing to watch out for with omni's is loosing the phantom middle section. Often the vocals are sent to the middle of the sound field, while the instruments are spread throughout. If the mics are spread too far for the type of venue, you can loose the middle and have a lot of separation...this can sometimes leave the tapes sounding a little thin compared to what you are used to hearing with hypers/cards.
2-3 foot spread is commonly a good spacing, however, more can lead to just as good if not better results if angled properly.
Phil