Heather,
The one place to be certain in the Magic Kingdom to meet her is at
Cinderella's Royal Table. This wonderful dining experience is held within Cinderella Castle, offering what I think is pretty good food and far better character experiences.
This is now a character meal at not only breakfast, but also lunch and dinner. While "officially" they never commit to what princesses will be there to visit with guests beyond Cinderella, Snow White has been a regular seemingly forever. It is expensive, but I know for my 3 and 5 year olds it has been well worth it.
Beyond that meal, you can always check the tip board at the far end of Main Street, USA. They not only list ride wait times, but also character greeting opportunities and times. One might include the princess greeting area at the
Town Square Theater. Princess 'selection' at such a venue can change too, so Snow White might drop in.
Oh, and with the Fantasyland expansion opening up soon, and including a Snow White-themed attraction, I would not be surprised to see a Snow White meet and greet in the open areas there too.
Have a magical vacation!