When I ordered my wedding dress, I was in the middle of a big weight loss and wanted to order a smaller size. The people in the shop said they always order at the size that you are when you come in because you can't predict what size you'll be by the time the wedding happens. Some people lose but some people gain!
I continued to lose and when my dress came in, they had to remake it onto my body. I didn't mind though because for the cost of alterations, I ended up with what was essentially a custom-made dress.
A few months later I ordered a bridesmaid gown and the sample must have stretched out plus I had lost some more weight due to gallbladder surgery. Well, by the time the dress came in, I was 5 lb. heavier and it didn't fit at all. They let it out as much as they could but I was pretty uncomfortable and ended up never being able to fit into it ever again.
So based on both those experiences, I'd say get it the size you are now and let them take in what they need to when the time comes. It's a much better experience to have people exclaiming about how much weight you've lost compared to having them tsk tsk as they try to make something too small fit you.