Should old knob and tube wiring be replaced?
Generally, it is a good idea to replace it, especially if you can easily access it. One thing you don't want to do, is replace part of a circuit. Then the next guy might see the new romex and make some assumptions about the rest of the wiring. The other time you really really should replace it is if it runs through insulation, especially blown-in cellulose. That is a serious fire hazard. Knob and tube was designed to use open space as an insulator.