First, I don't have an electric, so take this for what it's worth, but here are a few things to think about:
1.) Water in the drip pan won't make the meat more moist. It may aid in steaming the outside, but adding liquid to the meat it will not.
2.) You really don't want anything making contact with your heating element. So if it's directly underneath the meat, then you really want some sort of pan in there at all times; though, you don't need to always put something in there. Anything you put in there will act as a heat sink, which is sometimes helpful to reduce temperature variation.