Thanks, guys. I appreciate it. I will go with the mortar mix. No reason to take unneeded risks.
Yeah, it costs next to nothing and is easy to do, hard to not do that.
When I did my tub about 10 years ago, the manufacturer didn't recommend putting anything under it, but I used a combination of mortar and spray foam anyway. I think I put them in alternating rows under the tub. At the time I'm sure I had come up with some logical reason for doing that, but I honestly don't remember.
Anyway, worked out great, solid tub 10 years later.
BTW, my tub was a combination of a thin steel layer over some thick lightweight composite. Feels like a cast iron once it's installed, but it was light enough I could install it by myself. Turned out to be a great choice as it has held up well.