Yes! You do need to model sealed enclosures, too! Sealed enclosures are much more forgiving, but it's still a good idea to model a sealed enclosure so you know how much power your driver can handle. Drivers with a high Qts or in an abnormally small box can also lead to peaking in the bass, which can make your design sound boomy.
The other important thing this lesson covers is how to model a sealed design with amplifier boost to extend the low end response of your subwoofer or woofer. Some plate amps, such as the Dayton Audio SPA500, have the ability to add boost in certain frequency ranges, and some have a built in preset boost, such as the Yung SD300-6. Using these correctly can result in a sealed design with bass extending down to 20 Hz flat, or if used incorrectly, could result in a big 6 dB peak above the nominal response.
Again, the software being used is the Jeff Bagby's Woofer Box and Circuit Designer Excel spreadsheet and the subwoofer being modeled is the same subwoofer used in my first two videos, the Dayton Audio RSS256HF-4.