Non existent. Target sent out two messages (once recently, and once back around October) saying that Starbucks should not be promoting Red Cards because it conflicts with My Starbucks Rewards (Starbucks gift card rewards). It's the one place where you are actively discouraged from getting Red Cards. A Starbucks gift card will save more than 5% in the long run if you use it a lot, plus most stores in my area only have one register at Starbucks, so it would tie up your only register for a long time.
Having said that, I don't see anything wrong with mentioning it. If a regular is consistently using a debit card, I might mention the Target Check Card, but that's it, and I don't expect my team to do that.