Good question Dave!
I had a similar issue when I thought my tub rocking was due a poor install of the stringers. So in the mist of trouble shooting my tub wiggle, I knocked down the stringers down a 1/4". Wasn't the issue, I found a bulge in the floor. To solve this I laid cement board under the tub area and followed with self leveling cement. My new installation of the tub went with no wiggle, so i hooked up the drain and overflow tube.... but i forgot to raise the stringer.
so like you, in the sense, I wonder what's more important. A level base or stringers?