I agree, if she wants to be a part of this and you don't mind, then by all means bring her!
You don't have to bring her to every single place you look, but it could be really nice for all of you to maybe go have lunch, look at dresses and have some ladies time together.
I think it's wonderful that you have a future mother in law that is involved and having real interest in what you are doing.
I suppose it would matter whether or not she ( your mother in law to be ) expresses any interest in helping you shop for a wedding dress. If she would like to be a part of that experience, it may be an opening for some bonding time, as she is now gaining you as a daughter into her family. I would say that if you choose to exclude her, just don't bring it up when she is around, but if she expressed some interest in coming along, I think that her interest should be acknowledged. Remember, you are the one that is going to pick and wear the dress, constructive opinions are useful. This is also a woman that raised your future husband, starting the marriage on awkward terms won't make life in the long run pleasant.