"Perfectly level" doesn't seem to exist, but you want to be as level as you can, especially front to back. You don't want the slides to be tweaked at all.
It also depends on the size of the slides and whether they're in a pair or not (where the left and right slide in and out together). If you're out of level from side to side, then the motor has to work harder to deploy the uphill slide and to retract the downhill slide.
We've run ours out without leveling up first when we couldn't help it. The smaller bedroom slide doesn't complain much, but the two large slides in the main living area audibly complain when they bind up.
Short answer: Level up.