For pork butt I use a large water pan with water. I believe that keeping humidity level up and temperature stable results in more uniform temperature rise in the meat (Without a stall). (I have no idea whether this makes a better product) It also seems to make it easy to go 7+ hours without touching the smoker. Pork on one or both grates.
For whole large poultry (capon or turkey) I use the pan dry to block radiant heat, but generally only use the top grate.
For ribs I use a dry water pan if just cooking on the top grate, wet pan if using both grates.
For smaller poultry or beef roasts I don't use any water pan, but only cook on the top grate.