Simple.......the main reason to use differential pairs, is to minimize the noise..correct? So if we need to minimize the noise, then what should be done?? Answer, The phase cancellation should occur...correct? How will phase cancellation occur on differential signals? Answer is, route them parallel to one another with the minimum DRC and match their lengths. Sometimes when the differential signals exit out from a pin, there might be a very small portion of the exit which cannot be routed closely to one another..well, thats ok.