Hi HenryLee
I use Thoth too - but never use reversals with it (I do if reading with RWS).
The majority of people I've asked say they do not use reversals with Thoth either, and practically every book I have read on Thoth suggests NOT using reversals.
The deck is such that it has more than adequate capacity to provide the diversity of readings, that it does not require reversals for interpretation. I believe Crowley himself did not intend for reversals to be used with the Thoth deck and books I have read by his peers suggest the same.
If you get into the books about Thoth that also explain links to the Qabalah, you will see that the Thoth deck is supposed to adequately cover all aspects of life as it is: hence negating the need for reversals altogether.
One area of Tarot study (of great importance) which DOES bring into play the negative/positive card meanings is that of "dignities" - which some often mistake as strictly meaning reversals. "Dignities" relates to aspects of the cards such as the numerological, astrological and elemental attributes they have and how these affect the surrounding cards. Learning this helps you understand how cards interplay with eachother and therefore get a cleaer interpretation of the card and the entire spread.
For example, Cups relate to the element Water and Wands to Fire - generally fire and water do not get along and so that one aspect of those cards could be perceived as meaning they are "ill-dignified": which would put a negative spin on the card (as reversals are sometimes seen to do).
I find this a far more accurate and solid way of defining the positive and negative meanings of any card within a spread.
There are few books on dignities to be found (at least, I've had little success), but I do find this one helpful:
"Tarot Decoded: Understanding and Using Dignities and Correspondences" by Elizabeth Hazel (available on Amazon).
Crowley's fabulous "777" is filled with card aspects and attributes but is quite a complicated book (goes heavilly into Qabalah) and generally only for the advanced student. Nevertheless, I'd recommend a copy of it be in any Thoth readers library - along with his brilliant "Thoth" book itself.
Granted, dignities can be a tougher subject to learn than reversals, but is an essential part of advancing in Tarot interpreation.
This slightly older forum thread may also help:
http://www.tarotforum.net/showthread.php?t=35222
Good luck with your Thoth deck.