When I have post nasal drip, should I use nasonex (mometasone) or allegra, or both? None

You. You can use either a steroid nasal spray such as Nasonex (mometasone) or Flonase or an antihistamine such as allegra, zyrtec or Claritin for post nasal drip. In many cases the best response occurs if you use both, which is certainly safe to do. Importantly, it takes a few weeks for the steroid nasal spray to take effect so you should keep with it to see if it helps you.
Either or both. Whether they would work or not is another issue but you can use both.