I am very interested in this as well. I am about to plant some sunflowers for fall harvest. I had planned to have them 12" apart thinking that this would get them close enough to shade out most weeds and help support each other. After reading your post it sounds like I may be mistaken. Maybe I would be better off just planting them in standard garden rows so I could mechanically cultivate between them.

For the record I am planting about a dozen varieties from Sundance Kid and Big Smile to Mammoth Russian and Kong.

I plant sunflower rows about 7 inches apart. The width of my hand cultivator (three prong hoe.) I cultivate weeds for the first 3 weeks and then never weed again. The sunflowers grow better than the weeds. I grew 1/2 an acre and never cultivated, nor weeded and they were beautiful. And - its not that weeds don't grow for me, its just that sunflowers compete well and I have decided not to weed them.