Both processes, C-41 and E-6, are a standard. They should be run the same way in every laboratory, concerning development of the film. C-41 depends more on the laboratory you chose if you want technicians to print it, because the interpretation of the negative is NOT standard. E-6 slides on the other hand are positive images and can be used as an actual reference for the print to look as closely as possible like the original slide. Obviously not so good laboratory can be less careful on replenishment of the chemistry or film handling (scratches or dirt), but in principle development is the same everywhere.