Tapered pipe threads such as NPT are not designed to seal without
sealant.  There are other tapered pipe threads that will, such as NPTF,
NPSF and NPSI.  These are also known as dryseal pipe threads.  The
thread form for dryseal threads guarantees that the roots and crests
make contact by the time the flanks come together.  Unless you know
which type you have, I would recommend a sealant on all tapered pipe
thread connections.
Chris Zavatson