Basically you should know three things:
1. In most cases Google .ca will have different results the main index
2. .Ca index includes sites with .ca or hosted in Canada and/or any site in the main index results though these seem to be weighted differently then in the main index ie: the .com etc. site often places lower than it does in the main index
3. If you are working with Canadian sites understand the .ca index is default and so if your clients are targeting local then it is more important to place high in .Ca index then the main index. Use the main index as the target but don't sweat it as much as .ca index.

I managed to get Google to default to .com but... can't remember how I did it.

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