A common misunderstanding is that NPT threads seal because "the tapered
threads jam together". That's not true, and a properly made up NPT threaded
joint will predictably leak unless some sort of pipe joint compound or tape
is used

Yes, I've found this out the hard way trying to plumb a shower head
without the tape. Leak city!

Lastly, what is IPS threading? The MSC catalog has a lot of brass
fittings and nipples marked as IPS sizing. Is this compatible with NPT
or just with itself?

IPS ("Iron Pipe Straight") eqivalent to NPS. Not really compatible with NPT.

There may be a case of multiple meanings here. In the last couple of
days, I've called MSC and a plumbing supply house, and they both told me
that IPS is "iron pipe size". The MSC customer support guy assured me
that all their fittings sized as "IPS" in the catalog do in fact have
NPT threads (ANSI B1.20.1). That said, however, I did find a couple of
references online that talked of NPS straight pipe as IPS. Perhaps this
is a case of the IPS designation refering to pipe size independent of
threading type? Or maybe a few things are confusingly called IPS for
different reasons?

-Adam
adam at airraidsirens dot com