Here's my advice after 30 years of working in fittings: Since it is a tapered pipe thread you can leave the Teflon tape off the first 2-3 threads where it will make more than adequate contact with the HX fitting. 3 wraps of Teflon on the remaining threads will seal if the threads old or new are prone to leak. Even if you put Teflon on all the threads it is pretty unlikely that running the plug into a taper with a wrench will not obliterate the Teflon on the first thread. Chance of isolation is pretty small but possible, say, if you screw it in with your fingers only.
If it leaks after this, remove, clean it off, and apply a few more wraps of Teflon tape in the same manner as before. This only has to hold water for a year or 6 months, so tighten just enough or you'll wear out the permanent part prematurely. If it leaks and you crank it in more you're just trying to make up for lack of sealant by ramming it in. This is a maintenance thread, not a permanent threaded connection, so don't use the old addage that you don't use sealant because tapered threads self seal. True, but if you do you'll need a new bushing on you HX in short order.
