To me (a drummer...), 9/8 doesn't impose any grouping per se, but it could well be written that way, and often is. It could also be simply un-accented 9 beats, with no inherent grouping at all and would still be 9/8. It is often more 'readable' to write each part in its own time signature, though, and have alternating bars of 3/4 and 3/8 in the example you give. Some would write as 4/4-5/4, too, instead of 9/4; the difference really is in the intention of the composer rather than any strict 'theory' or 'rules'.
I'll have a lait-fraise if you're buying...