Thanks. I have the flange spacers so now I can do it either way, spacers on the subfloor or on top of tiles. I'll let fear be my guide once I start tiling the floor.
We took delivery of the tub(American Standard Princeton). The instructions call for a 13 5/8" high 2x4 string across the back wall. I set up a laser level and found the highest 13 5/8" measurement based on all 4 corners of the tub and then used that for my height. It also didn't call for stringers on the two shorts sides, but the old tub had them, so I put put them up to. Is there any reason to NOT have stringers for the front and back as well?
The plan is to "dry fit" the tub, shim the apron as needed and then for the final install set the tub in a bunch of piles of mortar that will be squeezed down so the tub sits on the strings and apron shims set up during the dry run.
Here's a photo of the tub stringer with the laser level: