Have you ever heard or seen anyone set a new shower pan with roofing tar ? I've always used caulking or mortar but I'm not a builder. Anyone have experience with this?

I've used spray foam in the cans before under fiberglass tub and shower bases to support more weight. Once the foam sets up there is no give for the fiberglass to crack when someone is standing inside. I've also seen people use joint compound that you use to finish gypsum board. I've never heard of or seen anyone using roofing tar though. Some of the newer shower pans and tubs are made with 3/4 inch plywood fiberglassed in underneath for extra support, but if a floor is out of level, you still have to support it anyhow when you level the tub or shower pan.

It used to be common to use it as a floor sealant. 
I haven't seen it in any remodeling shows on TV, though.

I have done a couple remodels involving restoring hardwood floors - after removing rugs and linoleum.  And there's almost always roofing tar sheets under the linoleum.  I'm sure it kept water out great ... But it's a royal pain to remove. 

I also did a remodel of my bathroom, and there was roofing tar paper under the tub.  Thankfully, I was installing a shower and didn't have to remove it, that time.

It was called "hot mopping".  They would bring in a bucket of tar, heat it up and use a mop to apply a liberal coat of tar to the shower pan areas and up the walls for a foot or two.  Don't recall if they did that or not for our bathroom remodel. 

It was used to waterproof the pan area many years ago. There are much more modern and better ways to waterproof today. I don't think any builders use hot tar any longer.