Personally as a feral tank I dont like to use agi/stam gems, trying to stick with purely one or the other. Id make sure you have ~30+ expertise before you toss agi gems into red sockets. I recently switched some gear out and I had to switch to some 20 expertise gems to keep cap in my feral dps gear and to keep myself at 30-33 as tanking. Ideally you want to have 40%+ dodge with 0 buffs sitting in bear. Once you have that you're free to gem for more stam or agi, whatever you would like to.
Edit: Gaffigan being my druid tank :P