Most websites or calculators don't handle tanking well. For dps, they can be pretty good. For healing, well, you definitely need to use some of your own judgement. If you are running out of mana, you need more spirit no matter what a website says.
For tanking, you have entered another level of complexity. There is a huge intertwined mess of both your damage reductions and your healer's capabilities. Also, your dps'ers affect that equation because the less damage they can take, the more the healers can focus on tanks.
That kind of complexity can't be fully modeled well. The excellent theorycrafters here tend to look at one facet at a time. They can look at rotation and tell you rage per minute. Or look at a fight and tell you about total damage reduction or damage spike prevention. It's not usually possible though to overall say that THIS is the one way to gear or the exact sequence of ability use that you should use. That's why a site like askmrrobot should not really be relied on for that kind of info. It can help you get hit capped and expertise capped. It can help you decide if you want to go with the non-tier legs or shoulders. It can be used in limited ways for tanking. You'll need to use a lot of your own judgement and research on top of that, however. Also, it is often a good idea as a tank to have options in the bags, especially when it comes to trinkets.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng