I live in Southern Ontario. Will African Violets survive our winters in the garden, or do I have to take them into the house for the winter?
African violets (Saintpaulia) will definitely NOT survive outdoors! They need warmth year round and can't grow outside anywhere in Canada or the US, except for possibly Hawaii.
You can grow many kinds of traditional violets (Viola) which would be hardy. Note that even some of these may not be reliably hardy in your area, but many are.
Here's a link to gw's African Violet Forum.
Here is a link that might be useful: African Violet Forum