Do I need to serially dilute E. coli cultures for optical density measures?
One is not normally required to serially dilute E. coli cultures for spectrophotometric measurements, at least in the experiments where the OD value is important. 

For most protein expression work, the consensus is to start the induction at OD 0.6, and for most chemically competent cells, the optimal OD ranges from 0.3 to 0.8 depending on the competency protocol. At no point in time do you need to perform dilutions, as they are all within the linear range of cell count:OD ratio. 

The only time you need to dilute the culture is when you want to find the OD of a very concentrated culture (for example after 24 hours of vigorously shaken incubation).