I bought my dress back in May. It was a size too small (off the rack and clearance) but the consultant and seamstress working that day both agreed that it could be let out. Deep down I knew I was making a mistake, but since they are professionals I went with it because I love my dress and it was in my very small budget. I lost 12 pounds since that time so I thought maybe it would even fit and not need to be let out. My boobs were the first to go, of course.
I went back and forth on using Davids or a seamstress who owns her own shop. I am glad in the end that I went with Davids because they had to make it right since I bought the dress there and paid for the alterations there as well. 99% of the time I am a small business type for girl.
I should have listened to my gut for this process and kept looking for a dress that wasn't too small. I had a feeling that they were not correct in being able to let it out enough.
Alterations story in comments.