For a sealed box, they're not required. The subs have a natural roll off according to their frequency response and the airspace of the chambers within the enclosure. For vented/ported boxes, setting the subsonic filter below the box's port tuned frequency increases your chances for unloading, where the subs have little to no control and things start getting very smelly. If your equipment has an accurate crossover, then you should be able to tell pretty quickly while tuning. Hope this helps.
1998 BMW 540i
Headunit: Stock DSP; Hifonics MX-1
Front Stage: Cerwin Vega 5.25's
Rear Stage: OEM 5.25's
Sub: RE Audio SX 10.4
Amps: Rockford 400.4 & bd1000