as long as both ends of the cable are wired to the same spec it doesn't matter which one you use. If you want to make a crossover cable just do one end 586A and the other end 586B. Functionally A is no more backwards compatible than B. It is a good idea to stick to one standard throughout your network but that is just for the sake of conformity. As long as both ends of a cable run are terminated to the same standard that section of cable will function correctly no matter what standard was used to terminate other cable runs in the network. The only difference between A and B is the pairs used to carry the TX and RX signals. doesn't matter what pairs are used as long as they go to the same plug contacts on both ends of the run.