In friction shifting, it doesn't matter much, as long as the derailleur can sweep the full width of the cluster. With indexed shifting, you have to be sure that the amount of cable pull each shift requires matches the travel of the derailleur, or you'll never get proper indexing.
Shimano has changed the cable pull specs between models and over the years. I'm not sure that a 6-speed 105 derailleur uses the same cable pull as an 8-speed STI lever.