Is it acoustic you're recording or electric? If it's acoustic, I always ran in to problems recording it with just one mic (the fact that the mic was an SM58 going in to my 1010LT's pre didn't really help matters either).
I'm really no expert on this, but I think most engineers like to use 2 small diaphragm condensers, 1 at the 12th fret and 1 at the body (away from the sound hole - too boomy). I don't know whether you'd consider this to be 'stereo' as such though (say like M/S recording or X/Y or whatever) - it's more about capturing the tone than creating an accurate stereo image. If the omni has 2 pres (which I think it does) and you have 2 mic's, you could use this method. As I only have the one mic though, I've had work out a different way. What I do is DI my electro-acoustic in to one of the pres and then mic the guitar at the 12th Fret with the 58 and have that go in to the other. I can then record them on to separate mono channels and play around with the levels of each to get a nice mix. I guess there could be phase issues but so far I've not run in to any. The improvement in tone is quite considerable as well.
If it's electric you're recording, most people you speak to swear by a single 57 angled towards the edge of the speaker cone of your cab. If you want to do more than this, you could stick a large-diaphragm condenser about 6 feet away to capture the sound of the room. I think top engineers also like to turn up the levels until the speaker is being driven really hard and the cabinet it's housed in starts to vibrate and have a positive effect on the tone.
Hope this is helpful anyway. Like I say, I'm no expert, so if anyone else wants to offer corrections, I'd be much obliged!
