A KM did address this matter. It stated that each individual must decide based on circumstances, which congregation to attend. But, if there are no good circumstances, then they will emphasize that you go to the one you are assigned to. Some congregations in Canada are so packed that they give you the option to go to another one. I don't exactly remember where that KM is.
Servants who don't go to where they are assigned, will be dealt with, and you may not receive any privilledges, if you attend an unaasigned cong.