yo i just wanna tag in here real quick to say that you don't need infinite EH to tank bosses, you get enough and then going for avoidance (aka mitigation) is smart. personally, with 30% buff i always play with SM and at least couple of dps items on my gear for better threat in general, but when it comes to no buff content i always equip highest EH set for lich king and sindragosa (because these are the only bosses which can, realistically speaking, 2 hit you to death from full hp on a no buff run [also technically speaking bpc when tanking valanar and taldaram]).
in fact, i use very simple rule for this
>if boss can kill you in 2 hits from full hp, then play serious gear and don't joke around
>if boss can kill you in 3 hits from full hp, equip some mitigation/avoidance or damage items for threat (up to 3)
>if boss can only kill you in more than 3 hits from full hp, you may equip 4-5 dps items (trinkets, weapon, some offpieces like rings or wrist for higher hit/expertise) or avoidance items for higher mitigation (again, trinkets, offpieces which provide more overall avoidance which means less overall dmg taken)
again, you don't need same EH for all 12 bosses, the only serious bosses you may want to respect are sindragosa and lich king (and tanking 2 bpc bosses), especially if you don't have great items yet.