On making gravy..
As I just mentioned, I consider the omnis + a forward facing center mic the basic meat & potato core of this technique.
From there the next step towards sonic deliciousness and increased flexibility is the gravy addition using any one of these things:
1) Turning the center mic into a M/S pair by adding a coincident bidirectional.
2) Substituting an X/Y pair for the single center mic.
3) Adding a rear-facing near-spaced center mic.
4) Adding near-spaced, 180-degree opposed sideways-facing supercardioids.
For most tapers I'd suggest the first two options. They are familiar, and sort of direct extensions of what adding a center directional mic to the spaced omnis does by addressing what is most deficient in a pair of spaced omnis on their own.
Option 3) is sort of more of an extension and way to fine-tune what the omnis are bringing to the table. It's more about the natural sense of space, ambient depth, the "you are there" stuff, rather than upfront stereo imaging, yet still helps glue the center and sides together up front, just not in a sharp stereo imaging sense. It's a great addition to the other 3 options, yet useful on it's own without them. Still I think I'd recommend the first two options first for most tapers. [edit- Fans of straight up spaced omnis and "you are there" imersion junkies might do this first though. This was what I ran for quite a while before going further exploring these other options.]
Option 4) does both those things. Like a M/S or X/Y center, this expands imaging across the center. Difference there is sort of the near-spaced pair verses of coincident imaging thing, except this retains the center mic position which I feel is a key element, and instead of a normal 2-mic near-spaced pair it's a "3-way near-spaced pair". I really like what this does, it's quite magical in providing a sense of up-front presence and excitement for the main, primary-focus sound. [Edit- at the same time, it provides a more ear-perking sense of "you are there" imersion, with a true sense of direction for the audience, ambient reverberance.] For my uses, this option is more important than using an X/Y or M/S center because I'm using this to provide discrete 3-channel front stereo for surround playback, while it also works well for mixing to 2-channel stereo. Unlike the other options this requires a 5th channel since it's adding two mics instead of one, so a recorder with more than 4 channels is required. I don't find a compelling need to do this and use an M/S or X/Y center pair, instead I'd add the rear facing mic as the 6th channel.