We have a slope of 1.5" from front to back of the length of the tub for the concrete to drain, but it's level side-to-side. The water is just above the rear jets (the higher side of the concrete), which puts it just below the headrests on the front (the low end of the concrete). I was concerned before the tub was delivered, and asked the delivery guys too. They said to use it, and see if it is workable to keep the water level above the rear jets at the lowest point. If that's working, then there's no need to level it. If that's causing issues with usage, then the only option would be to level it out.
Basically, from what they relayed, the levelness has nothing to do with keeping the tub equipment working well, etc. You just want to make sure that it works for where you want to sit. The guidance that they gave was to make sure that water was 1/2 way or so above the skimmer, and to make sure that the jets were underwater so it's not spraying everywhere...
The delivery guys also said they'd installed them in much worse grades...not that their stating that means that it's a good idea....