I had good experience with creating tangent maps for my models (which being organic would really suffer from bad tangents) while also using UV mirroring for extra texture space and better resolution.
The way I do it, is I create one half of a model, UV map it, texture it etc. Then I symmetrize the whole thing, create a blank copy of the mesh (extract polygons[keep]) to clean its stack while also keeping the old model for reference.
On the new copy I add two basic texture projections, doesn't matter what orientation or kind. Then I copy/paste the UV's from the old model to the new texture sets in the texture editor.
Once that is done, I add the tangent operator and select the second UV set for it. The I go into the texture editor, select the second UV set, scale it way down and simply align/rotate all the UV islands to that I get a mostly green shaded model with as few rough transitions as possible (I use green because it works with my bump mapping, though the idea is to get even coloring and smooth shading transitions). With mirrored textures it is possible that you will sometimes get color transitions from left to right, but as long as you don't end up with sharp edges, you should be ok.
Then, I go to the operator stack of the model and select the TangentOp2 operator (Selection->Clusters->Texture Coordinates->Tangent->TangentOp2). There I crank up the smoothing all the way up to blur any remaining rougher edges.
This produces a relatively ok tangent map - I never have any shading problems in game, though I haven't yet begun to port models over to Rebellion. It is also fast to do, usually I'm done in about 20 minutes and that only because the old XSI 6 is so damn slow when editing tangents, otherwise I'd be done in 10.
A more detailed version of the above technique involves using an auto unwrapping method based on edge selection - what you do is, when you have finished that half of your model, you select your cutting edges and auto-unwrap the model, texture it etc.
Then, you symmetrize it, add another unwrap operator and this time select again all the edges for cutting; they should already be selected when you press "E". This will produce another UV map with symmetrical, non-overlapping UV islands, which you can then manipulate to get a truly proper and smooth tangent map. Takes more time though, especially if you have lots of smaller UV islands, though the Unwrap operator does a good job of eliminating most texture distortions even if you do large UV islands on uneven geometry. Although I think the Unwrap operator has been introduced only in Softimage 2011+, not sure that the Mod Tool has it.
This second technique, however, requires a more extensive set of tools and thus isn't appropriate for everyone. You need a polypainting app to perfectly match the texture distortions produced by the Unwrap tool, for one. It is however, extremely fast - you can UV map a complex model in about 10 minutes, including tweaking the UV islands to optimize texture space usage. Anyone who ever UV-mapped a model knows what a time consuming chore that can be.