It's not a bad idea to install a WH in a drain pan, but no, with the new federal requirements, WH generally do not have to be raised up. If the floor typically got wet, you might want to raise it for that reason. Note, some locales require it in places like garages, but the new federal requirements make the new designs safe from gasoline vapors, so the added height does not add safety in that front, but still does from possibly being hit by a car in the garage.
Check with your local inspector and follow the installation requirements. If the two don't agree, do what the inspector says after showing him the instructions - he may relent, and he may not.