Make it level as you can. If it's a hair off, it's fine so long as it's resting solidly on the floor and stringers. Take the time to test fit the tub with the stringers in place, and when you got it right, secure the stringers nice and solid. If you don't, it can creak when you step in it and it will drive you nuts trying to correct it after it's installed.

Also, it's a good idea to put a piece of cardboard in the bottom of the tub. Better to step on that while testing for squeaks and creaks.

Thanks for the good comments. This tub (American Standard Monarch) is an acrylic fiberglass. It is made to rest on the skirt at the front. It then has a horizontal wooden panel fiber glassed to the front panel that extends under the bottom of the tub and fiber glassed to it as well. The back edge of this horizontal panel is fiber glassed to a vertical wooden strip which acts as a long single rear leg. If I level the top of the tub it appears I have about a 1/8-1/4" gap under the rear leg. Any suggestions on how to shim this gap, given that as soon as you lower the tub down there is no way to get in there or even see what is going on.

I was thinking if it will fit is a thin strip of a panel with PL400 on the bottom of the strip, and nothing on the top so it will not stick to the tub support. I would put enough of the PL400 so it sits high but have the excess squeeze out under the weight of the tub.

Your thoughts? Good plan or ??

Just trial and error to shim to fit. It's not likely to go anywhere once it's in. Just make sure not to shim it so the stringer isn't bearing full weight all the way across. If you get a gap between tub and stringer, not a good thing. Good luck.