BBQer wrote:...I thought I might have been mixing up the idea of rubbing ribs with mustard and making BBQ sauce as to which used the cheap mustard.
I don't use expensive mustards for either. In the case of barbecue sauce, unless the mustard is the feature ingredient, the finer points are going to be lost in such a strongly flavored and highly seasoned sauce, so why bother? Aside from knowing that you used a fine mustard, no one is really going to be able to tell much difference. In the case of using mustard as a rub, most of the volatiles in the mustard blow off between 70(F) and 90(F). For a piece of meat that is going to slow cook for hours, most of the character of the mustard will be lost early in the process, so why bother with a pricey product? A basic, inexpensive version of that style of mustard will do just as well.
It's a completely different question for a mustard being served at table or added at the last minute. I make a really nice, spicy, German-style whole grain mustard for putting on sausages. And I have a 750-yr old Catalan recipe for mustard that will knock your socks off...