Flytyer provides some sage advice on tying multiple flies of the same size and configuration. Laying out the material in advance is also a good technieque used by the profesiionals. One other advantage to tying 6-12 each of one pattern/size is that with quantity, the quality improves. My rule of thumb on new patterns is that it generally takes tying one fly six times to get a constant quality.
Consistency also is a function of paying attention to detail. As an example, when I get materials ready for a particular pattern, e.g. clousers, synthetic material length and quantity are identical for the batch being tied.