I have a little pitch on my slab where my tub is and it is not an issue - little pitch to one side and then the one corner. As long as you can keep water just above the jets on the high side and go in without water spilling out over the pitched side, you'll be fine. For our tub, I positioned our filter in-take where the slab starts to pitch a little - although our tub is round, so I have total flexibility in spinning the tub into whatever position I want with regards to positioning the filter.
If you scroll to the 3rd photo below, you can somewhat see how the tub slightly pitches to the right and it pitches down towards the bottom of the photo as well. No problems, the concrete is solid and flat across the surface, which is important. Now, you if you have some massive pitch, then that could be tricky, as water may always empty out over that side when people enter the tub and thus, leave your high side too low with respects to some of your jets.
http://www.whatsthebest-hottub.com/forum/index.php/topic,16101.0.htmlJ
