One of the greatest Fuqaha (jurists) of the Shia, Sheikh Al-Adham Al-Ansari (may Allah have mercy on him), in his book, "Risalah Al-Taqiyya", explains that the usage of Taqiyya (dissimulation) can fall under five categories: Wajib (obligatory), Mustahab (encouraged), Mubah (permissible/optional), Makruh (discouraged), and Haram (prohibited).

So it is not as the opposers of the Shia make it out to seem - that the Shia are obliged to conceal their faith at all times! Each circumstance differs, and every situation has its ruling.