Well there are more problems sometimes with tub showers than kids splashing water out. With a curtain (or even glass), water can dribble down the corners of the tub and collect in the corner where the tub meets the floor. If this is not waterproof, or it sits against drywall, rot will eventually occur.
What I do is use Ditra on the floor as tile underlayment. Up against the tub, I use a strip of KerdiBand, and use a polyurethane sealant to attach the edge of the KerdiBand to the tub. Make sure to get the corners, as mentioned.
This way you are not relying on any surface caulking to waterproof that area - it's waterproof from the "inside" so to speak.
Regarding the level of the floor, I'd prefer to have it level if water is getting splashed out there. You really don't want it collecting in any particular spot (unless you're going to go Full Monty like JazMan mentioned and build in a floor drain.)