You are always best to level as much as you can but putting lifts (boards or lego blocks or anything) under the tires first, then level front to back, and then deploy the slides. This method will never hurt anything.
If you actually lift the trailer with self leveling automatic jacks, then there could be a chance of the frame twisting and then deploying the slides might not be so nice. But, you are still best to check with your dealer or manufacturer for your specific camper.