I just got my i545 and had the same issue. The new phone (bought off Ebay) has MJ7 and the 4.4 update was beating me to death.
I rooted with the method in the General forum sticky. Then, I paid for the Titanium Backup registration key. With that, I could freeze FWUpgrade.apk and SDM.apk. No more prompting.
I added Safestrap after that. use the 3.71 version linked in Hashcode's thread (Safestrap-JFLTEVZW-3.71.apk). With this, I could get the 4.3 to activate Hotspot tethering ( I am still on grandfathered Unlimited). Voila, everything works just the way i want it to.
I am unsure if I want to go to 4.4 at this point. I would eventually like to have CM11. But, with 4.3 and tethering working, I am pretty satisfied with this phone already. It is miles ahead of my HTC Thunderbolt.