Optical density as a function of bacterial cell size

[billing at vetsci.microvet.arizona.edu]

At what point does bacterial cells size (say, rod vs coccus) 
significantly affect optical density (say 550-600nm)?  I guess what I am 
asking is, if 10(7)cfu/ml rods have an OD of X, then will 10(7)cfu/ml 
cocci have a similar OD or will it be less, due to less light 
scattering, etc? 
In theory it should be yes, but in practice is the difference between 
E. coli and S. aureus going to be significant?

And while I am on this thread, I was always taught that turbidity could 
not be eyeballed until the culture was about 10(6) cfu/ml.  Has this 
"myth" been perpetuated anywhere else, or does it too, depend on the 
bacterial cell size?

TIA

Steve