Been a long time since I've installed a window AC and actually looked at the instructions... I remember the one I put in my apartment in Washington, DC, actually had a platform that you had to assemble that it would rest on and that kept it level, but that was largely because it was for a casement window and it needed the platform for support.
The ones I put in my Mom's house... I just threw them in the windows, and if they were level, great. If they slanted a bit, great. I didn't much care, as long as they didn't drop out the window and go through the porch roof.
What you do NOT want, however, is them pitching towards the interior of the house. They do that and you'll start throwing condensate back into the house through the vents.