I don't see any problem die cutting a taper, I guess the only tricky bit is ensuring the taper die is not driven to deep or the threads will bottom out before they seal.
From my engineering handbook, the gauge plane and diameter at that plane that your aiming for are (for external threads)
for the 1/4"
Gauge plane 0.237" from end face
Major Diameter at that point 0.518"
for the 3/8"
Gauge plane 0.250" from end face
Major Diameter at that point 0.656"
I would guess the complete, incomplete and washout thread will dictate itself given the above depending on the OD of the pipe. For reference I've never heard of the taper being pre machined and I know the guys I work with just cut straight on to parallel pipe! I'm certainly not suggesting its the right way to go about things, but it seems to work for them.

Edited By Russ B on 01/05/2015 14:53:58
Edited By Russ B on 01/05/2015 14:57:07