DU does monitor rates and will try to secure you a lower rate if/when one becomes available. We have ressies at the Dolphin for this Chrismas and my agent has been absolutely wonderful. I got over anxious when I found a lower rate on-line. I went to cancel my ressie with Dreams and he actually got my lower rate for me and pulled in some favors to cancel the rate I had secured on my own because it was actually an internet - non-refundable rate. Just this past week, lower room rates were available and he was able to save me $5.00 more dollars per night. They have been absolutely wonderful to work with and I really love the Priority Seating Service they have - it saved me hours on the phone with Disney and I got virtually everything I wanted.
I highly recommend them and will not plan a WDW vacation without them!