So I've been following the instructions for making mpg's out of
PP+Aspect HD:
(1) Export to movie using Cineform HD Export (first_file.avi).
(2) Import first_file.avi into an NTSC project and export to
MPEG2-DVD.
These steps have been working well (although sometimes I've had to deinterlace the mpg because I haven't liked the appearance of jagged edges which sometimes result when non-interlaced -- but this is not the subject of this post.)

I'm seeing where the movie file (first_file.avi) output from Cineform HD Export is not always acceptable to other software packages. For example, Sorenson Squeeze 4.2 tends to crash when I supply it with such a file.
So my steps have been to take first_file.avi and next export it to movie using Microsoft DV AVI export, in order to create a second AVI file (second_file.avi). Then I am able to run Sorenson Squeeze on second_file.avi.

Since I clearly don't understand what Cineform HD Export is doing,
I have some questions:
(1) Is there a codec that will allow other software packages to read the output from Cineform HD Export without going through a second step?
(2) Since I'm creating an intermediate Cineform HD Export avi file, is this making the final DV AVI file better (since Cineform HD Export is in the loop) or worse (since I' m double-encoding to AVI)?

CineForm HD Export is simply the best way to create a CineForm master AVI, this is compatible will most PC tools. Note: Tools are aren't compatible are due to those application not supporting Video for Windows or DirectShow sufficiently (the CineForm codec is registered as a standard codec under these protocols.) However you can encode directly within Premiere to any other codec if you wish, however it is best to avoid DV as it has a lower color resolution. Premiere only has issues scaling HD to SD if you use the Abode Media Encoder, anything that uses Export->Movie should work well.

Thanks for the reply, David -- it makes more sense now. Hopefully other software appications (such as Sorenson squeeze) will update their products soon so that I don't have to make this intermediate step.