The problem is not so much with Rhino to Softimage, as it is with iges to softimage. Most of your problems "will" be solved by the info - mental ray dialogue box - up the nuber of uvsteps.
The other problem - is with trims in general - It's a complete pain in the ass, and I for one haven't figured out a surefire way of fixing it - although it does not always happen.
One sort of roundabout - and shitty way of fixing it, is by tasselating all of your geometry after a texture has been applied using uv coordinates - this will preseve the uv of the surfaces - just dont merge them - theoritically - If you have finished modelling, and with render tasselation in mind, it makes no difference to have polygons there - although there are a few drawbacks in terms of rendering flexibility.
Keep in mind that when non uniform - adjacent surfaces are stitched in any way a seam - even if it is microscopic will show - as nurbs always tasselate. (unless renderman is involved)
Cheers - If ou work it out let me know.
- Non smokers die every day.