Micros doesn't calculate tax on the full price of an item"
That's not accurate. Micros can calculate the sales tax on either the total sale or the discounted sale amount, it all depends on how you set it up in the discount definition to effect Tax 1, Tax 2, Tax 3, Tax 4 etc.
"Micros would store the sales tax value on discounted items?" I didn't understand this part; it appeared that they are asking if Micros can tell you how much sales taxes were reduced; which I doubt is possible unless you have a single discount button, but I also didn't understand why anyone would want or need that info.