User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-6951062-20130312053730/@comment-3146930-20130312182331

Not exactly. While gods can open up doorways to the Underworld pretty much whenever they want, the LA entrance and the NYC entrance are simply the most popular ways for demigods to enter the Underworld. For example, in The Demigod Files, it is mentioned that there are a lot of escape routes that souls try to use to free themselves. Even Persephone admits that:

"''No prison is airtight, not even the Underworld. Souls are always finding new ways out faster than Hades can close them. You must retrieve the sword before it leaves our realm, or all is lost."

So there are other ways in and out besides those two entrances. The portal to Tartarus Arachne lived over seemed to just be a portal directly to Tartarus independent of any kind of being, godly or otherwise, making it. Based on this, portals in and out of the Underworld could just be harder to find and those two in LA and NYC and the only reliable ones, as Hades is constantly closing new entrances that are found. A portal out of Tartarus however will be harder to find.