Hi Jay,
I think from a purist view (which I strongly subscribe to) the less manipulation of the signal the better. In the case of NAB you're eq'ing the bass as well as the treble effectively doubling the number of resistors (in a passive circuit) of an IEC eq circuit. Even the best resistors have some coloration and while this might be insignificant with some lesser music source, I don't doubt that you could perceive their colorations if you A/B'ed different resistors with a TP tape.
Another thing to take into consideration is that this eq'ing is done once in the recording process and again in during playback.
On the other hand, I don't believe that these small differences would be as noticeable when recording a source which has already been thoroughly processed like LP's or CD's. I think that the quality of your phono playback (and lp cleaning before recording) or your DAC is far more significant to the finished recording than the eq that you'll use. The tape that you use will also determine the "character" of your recording a little so experimenting with the available tape types to find the one that you prefer should be done.
If you are planning to do "live" recordings, you'd probably want to eek out every last bit of fidelity you can. But here again, the quality of your mic's, microphone placement, and mic preamps will play a bigger part in the final product.
If you guys haven't noticed it, I'm a big believer in optimizing what I have at hand (or maybe I'm just plain cheap!). And while there may be a few lp's or cd's that might benefit from the use of another eq (other than NAB), I don't think that it's worth the extra trouble.
