Trade District
Darnassus on my main (Nelf Druid) as it is so peaceful compared to SW (and the cloaks etc let me portal around anyway) and SW on all other characters for ease.
Got my hearth set in the bar in Dwarven District. Everything I ned is right there, and its fairly quiet
I always set my heartstone around somewhere I am questing or farming something. Mage ftw :P
Dalaran on all of my toons, even under 60 ones . Most useful place to be at always.
Dalaran, got access to Northrend, fastest route to Dragon Soul via CoT portal and a Portal to Orgimmar.
Bounty Bay on my warrior.
Stonard for everyone else Role Play reasons
most are set to dal for ease of travel. port to sw right there and taking that gets you to outland easy enough. my mains is changed around alot, depending on what im currently farming with him, hes engineer with 2 of the sw port cloaks so he gets around easily no matter where i set it.
Shifting between Shattrath/Dalaran/SW(dwarf district) on my main (hunter), depending on what I'm doing at the time. If I'll be farming basilisk eyes for some time, I'll set my hearth to Shattrath, if the DMF is up I set it in Dalaran for back-up access to the Argent Tournament (nice spots for farming 250 ears while doing dailies that award tokens that you can trade it for the BoE pets). Otherwise I set it in SW dwarven district for the quickest access to everything I need with as few loading screens as possible. My main is a gnome engineer so I have easy access to Dragon Soul if needed by porting to Gadgetzan.
Dwarven District on all alts except my mage, who I park at whatever location is most useful. I'm still levelling it, so I pick the local inn as my hearth.
I'm not even sure I have a hearthstone on my mage... But the rest are a mix between Stormwind and Dalaran.