I'd shoot for level surfaces. I only say that because when you take off depending on wind or stick inputs, you could possibly flip your craft over as it will attempt to compensate for the unleveled take off surface. It can sometimes cause you gimbal to calibrate improperly on startup also, thinking it is at a level surface but when you get in the air your image will be off center. It's happened to me a couple times. Could've just been my situation though.
I had to land the aircraft and reboot on a level surface. Doesn't have to be a "perfect" level surface, but I wouldn't on a grade of more than 10-20 degree