On ribs I have found the rub will sometimes rip/tear on the bone side when I used mustard. I had a friend grab a finished rack with tongs and the whole bone side of the rub dang near fell off in one piece. After that I never used mustard again and haven't had a problem. Completely unnecessary and possibly makes it worse if you ask me.
I've also tried different oils on ribs and found them to be an unnecessary mess that ended up giving my ribs a greasy texture that I didn't care for.
FEC120, XL BGE, UDS, Cimarron, Weber Jumbo Joe, Weber Performer Platinum (Copper)