Yes, I respec my Cat before every instance.
Why? Because it's cheap, almost free (15s usually) compared to my own respec (50g sometime... /gasp) and it can really be the maker or breaker of your pet surviving. We all know that BM is tied to your pet's damage, and more importantly for my groups, the Ferocious Inspiration procs.
Here is my Cat before any resistances:

Growl 8 is free, Claw 9 is our main damage kicker, Dash 3 is a must, Greater Stamina 9 adds 320 health, Natural Armor 3 adds 160 armor and Avoidance 2 allows your pet to take 50% less damage from AoEs.
From here it's just a matter of where to throw those extra points for resistances.
A list you say? Sure! Below I have broken down each instance along with what Boss does what school of damage. If a Boss isn't listed, the do vanilla melee damage. Below each instance and Boss list I've listed how I spec my Cat for resistances.
Hellfire Ramparts
Omor the Unscarred - Shadow damage
Nazran - Fire damage
(Fire 3, Shadow 3 and add the rest to Natural Armor)
The Blood Furnace
The Maker - Nature damage
Broggok - Nature
Keli'dan the Breaker - Fire and Shadow damage
(Nature 3, Fire 2, Shadow 2 and add the rest to Natural Armor)
The Shattered Halls
Grand Warlock Nethekurse - Shadow damage
Warbringer O'mrogg - Fire and Nature damage
(Fire 3, Shadow 3, Nature 2 and the rest to Natural Armor)
The Slave Pens
Mennu the Betrayer - Fire and Nature damage
Rokmar the Crackler - Frost damage
Quagmirran - Nature damage
(Nature 3, Fire 2, Frost 2 and add the rest to Natural Armor)
The Underbog
Hungarfen - Nature damage
Ghaz'an - Nature damage
The Black Stalker - Nature damage
(Nature 5 and add the rest to Natural Armor)
The Steam Vaults
Hydromance Thespia - Frost and Nature damage
(Frost 3, Nature 3 and add the rest to Natural Armor)
Mana-Tombs
Pandemonius - Shadow damage
Yor - Fire damage (Heroic only)
Nexus-Prince Shaffar - Arcane, Fire and Frost damage
Arcane 2, Fire 2, Frost 2, Shadow 2 and the rest to Natural Armor)
Auchenai Crypts
Shirrak the Dead Watcher - Fire damage
Exarch Maladaar - Shadow damage
(Fire 3, Shadow 3 and add the rest to Natural Armor)
Sethekk Halls
Darkweaver Syth - Arcane, Fire, Frost and Shadow damage
Talon King Ikiss - Arcane damage
(Arcane 5, Fire 1, Frost 1, Shadow 1)
Shadow Labarynth
Ambassador Hellmaw - Nature damage
Grandmaster Vorpil - Fire and Shadow damage
Murmur - Nature damage
(Nature 3, Fire 2, Shadow 2 and add the rest to Natural Armor)
The Mechanar
Mechano-Lord Capacitus - Arcane damage
Nethermance Sepethrea - Arcane and Fire damage
Pathaleon the Calculator - Arcane damage
(Arcane 4, Fire 2 and add the rest to Natural Armor)
The Botanica
Commander Sarannis - Arcane damage
Thorngrin the Tender - Fire and Shadow damage
Laj - Nature damage
Warp Splinter - Arcane damage
(Arcane 3, Fire 2, Shadow 2 and add the rest to Natural Armor)
The Arcatraz
Zereketh the Unbound - Shadow damage
Wrath-Scryer Soccothrates - Fire damage
Dalliah the Doomslayer - Shadow damage (Heroic only)
Harbinger Skyriss - Shadow damage
(Shadow 4, Fire 2 and add the rest to Natural Armor)
Currently I'm doing Karazhan and Gruul's for raids, so they are included too:
Karazhan
Maiden of Virtue - Fire damage
The Curator - Arcane damage
Wizard of Oz - Frost and Nature damage
Terestian Illhoof - Fire and Shadow damage
Shade of Aran - Arcane, Fire and Frost damage
Netherspite - Arcane and Shadow damage
Prince Malchezaar - Shadow damage
Nightbane - Fire damage
(Respeccing halfway through Karazhan is not an option, so I usually get a sense of who we are going after and spec accordingly)
Gruul's Lair
Krosh Firehand - Fire damage
Olm the Summoner - Shadow damage
Kiggler the Crazed - Arcane and Nature damage
(Arcane 4, Nature 2 and add the rest to Natural Armor, this is because most raids have the Hunters work on Kiggler who has a nasty Arcane explosion and when your pet is tanking that, every little bit helps. Also, most raids have your pets steer clear of Krosh and Olm.)