Patch 7.3 introduced an undocumented change to heirlooms, which is that they now change primary stats to match your spec just like all of the other armor does. For example if you’re in a healing spec on your Paladin, the primary stat on your ‘looms will be Int, and if you swap over to a melee spec, then the primary stat will swap over to Str without you having to change your gear at all.
If you’ve been playing this game for a long time and already have a full collection of the heirlooms, then this change means almost nothing, except that you don’t have to bother with carrying around multiple sets of heirlooms while you’re leveling if you’re one who swaps to one spec for questing and another for dungeons/PvP. If you’re new to the game and haven’t completed a collection yet, then this change means you just saved yourself a small fortune in heirloom and upgrade item purchases. Potentially.
This is the part that doesn’t change, so technically if you want to be optimized to the teeth in all situations then you would still need to buy all of the various heirlooms to make sure you can cover that for the different specs. How anal you want to be about optimization is up to you. If you’re trying to save gold and don’t want to buy a full set of heirlooms, then the secondary stat that I suggest you focus on most often is Haste. If you want to be optimized all the time, then buy all of the armor pieces out there.
The other half of the class-specific heirloom guides are nearly done and will be coming out soon™.