A lot of bobs have a seperate 5v power connection so if your parallel port runs on a reduced voltage then you can supply the 5 volts from a separate power supply. One of the main reasons for people switching to smooth steppers is not so much the problem with the parallel port voltage but the operating system they are running, you need a 32 bit system for mach3 to work with a standard parallel port bob, those with 64 bit systems have problems with the parallel port drivers which is why they go down the smooth stepper route.