If you're running a ported box, you're going to need to bandpass the system to protect from unloading your sub. If you're running sealed, the LPF is all that's needed.
Jake - in theory, yeah, but some weird things would have to happen for there to be a problem. With enclosures typically being tuned 30-40hz you'd have to be playing 20-30hz material at or near full volume. It's worth keeping in mind but I don't think it's critical to have an SSF just because you have a ported enclosure.