pawain did awesome writeups on this
Some would disagree.
I prefer a pet with 80 phy, 80 fire, 80 energy if possible.
I leave cold where it is on a Cu and increase poison when possible.
I build my other general purpose pets with 80/80/ low cold/80/80 when possible.
All my Hiryus have 45 cold.
I fight in Eodon a lot. I do the Dinos for the Huntmaster Challenge often, so I like the poison resist.
I have pets that are built for a specific target such as Mel, Medusa, Corgul.
Those have resists that match the targets melee damage. I have never met a target that does significant spell damage to my pets. I think the only things I have not met are Paroximus and Exodus.
Unless the Devs decide to make high HP high damage mobs that do 50% cold 50% poison, my resist settings keep my pets alive. And if they do make that mob, I am covered on one of those damages.