My Alliance server Daggerspine has all the toons in Goldshire, even though it's a 2 building fire, Allies do it often enough to park all my toons there, and reap the rewards from one succesful fire quenching. Kharanos in Dun Morogh is a ghost town (heh, a pun), and though I've heard that Azure Watch in the Draenai starting area is easy, I haven't gone out there yet.
Darrowmere, my baby Horde account, has all the toons in Brill. Why not Razor Hill, since it's close by Orgrimmar? Because Brill only has the right side of the town hall set ablaze by the Headless Horseman, whereas Razor Hill has 3 buildings to quench. Needless to say, that quenching doesn't happen very often. Why not Falconwing Square by Silvermoon City? Yes, it only has one building front set on fire, but there's nobody there! It could be different on other servers, but I've noticed a tendency for nobody to go to the BC Expansion areas anymore.
A side note: If you do have a lot of toons, and don't want to have to worry about doing the town dailies more than during one login loop, do some preparation. I usually log in with my first toon, accept the "Let the Fires come" quest, then dump it and get "Put out the Fires" immediately. Go back inside the inn, and have all the other toons accept the "Put out the Fires" quest as well. Now, when you're logging in to trick or treat the innkeepers, and hear "My fires have died, left not a spark....." immediately run outside and loot the pumpkin, and this will also give you a completed status on the "Put out the Fires" quest.
However, this doesn't mean you need to turn in the quests immediately. In Brill, since the pumpkin and Matron are so close together, go ahead and turn it in and get your rewards, then dash back in the inn for the next toon.
In Goldshire though, you'll want to loot the pumpkin, then run back to the inn to get your next toon. With a lot of toons, seconds can count, and it's better to get the dailies over as soon as you can, because more and more people won't care to do them as the festival winds on. This philosphy would also apply to Razor Hill in Durotar, since the pumpkin lands straight out from the inn, and the Matron is a good distance away.
Now, as to the title of the post. The Sinister Squashling pet and the Hallowed Helm are needed for the Sinister Calling Achievement, which is part of the Hallowed Be Thy name Achievement, which rewards the Hallowed title after your name. If you don't feel like dealing with PUG groups to kill the Headless Horseman, you can also get them as drops from Trick or Treating the innkeepers every hour (usually 3-4 times a day for me, as I'd really rather be out questing).
Here's my luck so far:
Clockwrkmage on Daggerspine
Doomslinger on Daggerspine
Aosdanasaoi on Daggerspine
Tsuris on Daggerspine
Gwiddonyn on Daggerspine
Aosdanasaoi on Daggerspine (giving her the Achievement)
Bonescourge on Demon Soul x 2
Kukoshakaku on Demon Soul x 2
So, if you want something, get it, and you'll be sure to get it again.
I've also noticed a tendency to be repeat tricked - as in, if I'm a snake, and login an hour later, I'll most likely be transformed again, instead of getting a treat.
I know, I know, it's just numbers and statistics, but it feels weird to get your third Flimsy Male Troll Mask on one toon, without having any other masks received. The Random Number Generator (RNG) doesn't seem so random at that point, more like vindictive.