Be prepared to insist upon being entitled to preferred seating even when they tell you no. It's really a hassle, even if you get the US call center. And worse yet, expect to lose your preferred seats every time DL loads a new schedule, which they do a lot. I had to call 3 times to keep my exit rows for a BOS-SLC-PDX a few months ago. Perhaps this doesn't always happen, but be aware of it, and keep checking your seat assignment.