We just did this on our trip a few days ago. We had just 2 TS credits left for our 3and 5 yr olds, because we used 4 credits for California Grill, while our kids went to Chef Mickeys with the grandparents. Can't really send the toddlers without us to a sit down dinner!! So we used their TS for a QS breakfast rather than waste it..
In fact, Disneys own literature (that I saw at the concierge desk at the hotel), had a little "Did you know?" tip listed that you can use unused TS credits at QS.
Of course I would try your hardest to get any sit-down you can at the last minute to avoid wasting it...but just wanted to let you know that it can be done! Have fun!