Honestly in my opinion blue stamina gems should take priority over red agility gems for a tank set. You will gain enough agility and dodge from the leather pieces you choose for tanking, provided you are using the right ones. Keep in mind as well that if you end up with too much dodge your rage generating capacity will suffer noticeably. There are enough leather pieces, and to a greater extent accessories (rings/trinkets/necklace/capes), with straight dodge rating on them that you should keep agility as your second most important stat, making stamina your first. High health is a druid's best friend for tanking even with the removal of crushing blows (wow that was a long time ago) because the health nerf we took a few patches ago was fairly significant IMO and Savage Defense, while nice, doesn't truly mitigate an insane amount of damage. Granted, I wouldn't give up Savage Defense, but I wouldn't rely on it to mitigate so much damage that I let my overall health suffer when gearing/gemming/enchanting.
Just my two cents.