It all depends on what I'm smoking and what bark overtone I desire. For beef ribs, dry; for chuck dry, water, or demi with water; for pork ribs, dry, or mustard (usually yellow); For Butt, dry or yellow or a flavored Dejon; for pork roasts, dry or flavored Dejon; for poultry, dry dry dry; for Prime rib; water or demi with water. Just note that I add the mustard for a flavor overtone on pork and not as a binder.
Smoke particles will "stick" to oil or water (much less than oil) more and usually when people smoke their smoker isn't super dialed in when the put the meat on and a whole bunch of bad smoke gets "stuck" on the surface right out of the gate with oil....hence bitter acid flavor that makes your mouth tingle in a bad way!
If you want to have a firm understanding of how and why, take 4 pieces of chicken, season one up and let air dry in the fridge overnight, then take the other three (just before you put them on) and season one wet with water, one with mustard, and the last with oil, then put all three on the smoker at 225....This will earn you a first hand education that you can share.....