I agree with those in the larger spread camp. I like to get to the outside edge of stage setup, to be just inside the edge of the band on both sides of the stage, and somewhere in the middle area between the audience, pa, and stage setup. This is only true if you are also running a SBD mix of some sort to blend with later. Without that, it will take the perfect placement to pull a good balanced mix out of your omni recording. In that case, I would suggest Card or Subcard with a modest split of 1-3' max. As has been said, placement is key and IME it takes major skill to pull off really good 2-track stage tapes. Not only that, but the right band and right stage setup.
For bluegrass and small club shows, I usaully run my CCM5's in card setting and on the ORTF bar and clamp it to the front side of the center vocal mic stand or put up a small stand in front of that and just below instrument height, pointed upwards about 30 degrees. That seems to work really well for me and it blends well with the flat SBD feeds. Having the mics centered and so close to sound source usually reduces the phase issues of blending significantly.
Sounds crazy, but the best place to put some killer LD omni's onstage would be equal distance on either side of the drummer, about 5' high and right in the middle of the stage and band. I would love to find a band that will let me do that!
One time at a Del McCoury instore show at a local bookstore, they were using one mic and one stand since it was such a small stage. AT4033.
I clamped my CCM5's directly underneath the 4033 on the same stand, ran them card/ortf. Oh Momma!
The best part was that after the first song, the PA was crackling so they just turned it off and played the rest of the set unamplified. Out of habit, they kept playing around the mics just as they always would. Having Del lean in and sing directly into the Schoeps at 18 inches or less is one my all time taping highlights.
Don't be afraid to get those mics onstage. That's the best place for them.
Cheers, Phil