After reading several forum posts, watching lots of Pitmaster I did various tests using mustard, Oil(s), water, etc. and what I decided on was it the addition didn't really make any noticeable difference to me.
Put your rubs on early enough and the moisture in the meat takes care of it. I guess a little water isn't bad but all and all ... I just put the rub on about 30 minutes before meat hits the pit.