Always better to be safe than sorry.
Some stores will give you a song and dance about how you have to order your dress at least 8 months in advance, but I don't suggest you do business with these kind of companies anyways.
But keep in mind that if you order a dress size, not one made to your specific measurements, then you will need alterations, which also take time.
You also don't want to wait until the last minute and have any mistakes.
If you do get shafted by some bridal boutique somehow, it is entirely possible to take your dress to a qualified seamstress, or even just have them make you a dress from scratch, within a month of the wedding.
We had a problem with one of my bridesmaid dresses (It was waaaaay too small) and she took it to a seamstress who took it apart, and fixed it within one evening. (and made a matching hand bag for it... )
A full wedding gown would obviously be more complicated, but a month would be more than enough time for a professional to fix it.