Is warmogs good on jax?

Nah. Just get Randuins, and a Spirit Visage, and build damage for the rest, unkillable beast late game.

Depends on the situation... but Generally ... is pretty standard and situationally

If you needed help surviving burst damage or you are facing a siege team it's a pretty good item on any fighter or tank. Triforce and BoRK are the most popular damage items he gets but you don't necessarily have to build both of them. If you need to be more of a tank for your team, getting extra defensive items might be beneficial. He's more of an anti carry in teamfights though so his best defensive items are Randuin's and Banshee's so he can dive in on the enemy carry and wreck their face with a lamppost, but if you needed a little extra health to survive the initial burst of the carries, Warmog's isn't a bad idea after you get those items.