I've heard that it's better to have tourmalines with open C axes, how important is this in an opposed bar?
Tourmaline crystals can be a short, stubby shape or a rounded nodule, but by far the most common shape is a long, pencil shaped crystal. In all colors, the long C axis (parallel to the length of the crystal) is the darker color. The color across the short dimension is lighter in tone, and often a different color. This is called dichroism (meaning 2 colors).
About 95+% of blue tourmalines are too dark, and the c axis blocks all light (it is "closed"). In iron green tourmalines, perhaps 70% block all light. The other 30% usually have a yellow green or olive color which is not desireable. Chrome tourmalines (which are actually colored mostly by vanadium, and sometimes chromium), usually have an open C axis.
So in blue and most green toumalines, all of the reflected light comes from the sides. That is, the table is always cut parallel to the long C axis. The ends, instead of be cut at a reflective 45 degrees, are cut at a steep 70 degrees so that they don't reflect the undesireable olive color.
Pink, red and the rare orange tourmalines, on the other hand, almost always have the table cut at right angles to the c axis. That is, all pink and red tourmalines are cut to show the darker color. Sometimes a cutter will try to get more yield (a larger gem) by cutting the table of a pink crystal parallel to the c axis. In this case, he gets a pastel gem with lots of brown in the pink (a very undesirable color in my opinion).
Getting to your question, an emerald cut on a tourmaline with an open C axis will be brighter than with a closed C axis, but only if the cutter chooses to cut a reflective (45 degree) end facet. Why would the cutter not cut a reflective 45 degree end facet? When he does not want to reflect an undesireable color such as olive green, yellow green, or brownish pink color.
Also the opposed bar cut shows mainly square facets, and a reflecting 45 degree cut on the ends, introduces a v shaped facet at each end. So an open C axis is great for emerald cuts with a good dichroic color, but is of little value for an opposed bar cut that wants to show square facets. I hope I didn't lose anyone with this long explanation.