I believe the two fixes posted in

http://www.zen-cart.com/forum/showthread.php?p=371348 (for group discounts)

and

http://www.zen-cart.com/forum/showthread.php?p=366574 (for coupons)

are solid and should be incorporated into 1.3.8.

I believe that the defined constant COMPUTE_TAX_ON_POST_DISCOUNT_PRICE which is separately defined in each of these classes should be put in the Admin in Configuration->My Store so that they would only have to be defined once and could be reused by other discounting modules.

Thanks,
That Software Guy