For backward compatibility the driver also accepts the zero crossing signal on pin 17 as well as pin 12. However, some known rogue parallel port hardware cannot properly read input from pin 17. During the probe, the tw driver looks at both 12 and 17 to find which pin has the 120Hz signal. So it will handle connectors using either pin 12 or 17 transparently. You don't have to reconfigure anything in the driver if you change pins.
See /usr/src/sys/i386/isa/tw.c and the tw man page for for
details.