yep, thumbs touching is about as close as you can get while maintaining stabilty, expecially as weight increases. 9" wide is as wide as I get.
another way to look at is, is most olympic bars have smooth areas, and textured/cross-hatched areas. at my gym they look like this ( |||| = smooth, XXX = texture)
|||||||||XXXXXXXX||||||XXXXX||||||XXXXXXXX|||||||||
I'll grab right on either side of the center area, putting my pointy fingers right on the line where the bar goes smooth. its about 5-6" apart.
remember to keep the elbows close to the body on this move too.