Our company was part of the beta and we were having buggy issues when trying to use Stripe Tax alongside the Customer Portal, which AFAIK they haven't resolved yet. So just FYI if you implement and bump into this, it's them not you.
We shipped customer portal support a few weeks ago, apologies for not following up and letting you know! If there were any other issues though, do let me know -- kmoriarty@stripe.com
Hey! The issue we were having was related to customers being unable to change their subscriptions through the Portal with Tax enabled, which I just tested and still seems to not work. I've sent you an email so hopefully we can figure this out.