Yes, those seems to be overkill for PCB milling but if you build a mill it will not take too long before you start milling even other things. For PCB milling round rails are OK but don't use the unsupported type because they are just a joke and you will never get good enough results. I have built my first one with those curtain rods but it didn't take long before I decided to upgrade to fully supported type and those give me much higher speeds and accuracy.