My basement floor slopes toward a floor drain. Th actual drain is in the unfinished portion of the basement. I did nothing to level the floor and it has caused me no issues. It is a carpeted and tiled space and works great for a living area. I don't think it's worth the effort to level. My brother in law built up his floor to level using shaved 2x4's. He had a sewer backup which then deposited "black water" under his floor. He ended up having to tear out the floor to clean up the mess.