As far as tapered pipe plugs go in general, there are different classes of thread quality. The highest quality (rarely available) does not need any any sealant in pressure applications. "Standard" quality needs sealant. Some of the stuff the home improvement store sells will not seal regardless of what sealant is used in my experience.
the theory behind a tapered thread generally, is the sealing ability without anything applied topically to complete the seal. but the quality of the taper threads, material, and the threading process, all play into this equation. finding "quality" NPT threads can be quite difficult these days, especially in hardware store plumbing supplies, as well as the typical automotive NPT threaded item. in general, most will seal, but the "seal" will be "damp" without leaking. in other words, you will see dust clinging around the fitting, or it will be damp with nothing leaking or dripping. i generally use a dab of loctite 567 teflon sealant [part #56765] and wipe away the excess from the joint after it is tightened up. this stuff is high temperature, and works in gasoline [although i wouldn't recommend it's use in fuel systems, it works good in bulk tank applications],petroleum oils, natural gas [not over 300# pressure],butane, and propane, plus air compressor systems.
