For the most part, the vast majority of your gems should be +30 stamina gems. There are a few exceptions to this however.
Exception 1: If you aren't a Feral Druid, you must remain defense capped. That's 535 defense for heroics (Level+2) and 540 for raid bosses (Level+3). If you aren't defense capped, you would need to gem some to compensate up to the cap. Generally you would use either 20 defense rating gems (Thick King's Amber) or 10 defense rating and 15 stamina (Enduring Eye of Zul). You would like to use yellow sockets if you have to gem for defense in order to get socket bonuses.
Exception 2: Meta gem requirement. You can't meet the requirement for your meta gem with only blue gems. Tanks typically will use Austere Earthsiege Diamond (32 stamina and 2% armor, requires 2 blue and 1 red). You want to make sure your meta requirment is always met. This is generally done with an Guardian's Dreadstone (10 expertise and 15 stamina) in a red socket of a piece of gear with 9 stamina or more as a socket bonus. A Defender's Dreadstone (10 parry rating and 15 stamina) can be used if you find yourself too high on expertise however that is unlikely.
As a sort of general rule, if the socket bonus is 9 stamina or higher, it may be worth while just getting it, however the vast majority of the time gemming just pure stamina (Solid Majestic Zircon) will be serve you better.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.