I think it makes a difference what style dress it is. Mine is fitted at the top and looser at the bottom, I tend to lose weight at the bottom, so I was glad that I bought it in my current size. It fit just a smidge tight when I bought it, but even though I lost 15 lbs between buying it my wedding, it just made it a more comfortable fit. Alterations aren't cheap, I don't think I'd want to go into it knowing I was planning to spend as much on alterations as on my dress. To shorten the spaghetti straps, shorten the dress, and add a bustle (3 loops and 3 buttons) I paid like $200. I can't imagine how much it would cost to make it 2 sizes larger...and would it look right?
In the long run, we shape our lives and we shape ourselves. The process never ends until we die, and the choices we make are ultimately our responsibility.--Eleanor Roosevelt