Taking your vehicle down south for the winter? If it is a short trip it is likely considered "usual and customary use" of your vehicle. However, keeping it there on a long term basis however presents a different risk and your insurer needs to be notified.
Consequences of letting your insurer know? Possibly none or there may be a higher premium involved. Regardless of the state, a vehicle involved in an accident anywhere in the USA poses considerable challenges in settling the claim from Canada and especially so if there is injury involved and you're in a USA hospital.
Consequences of not letting your insurer know? In the extreme, possibly a denied claim for material misrepresentation or a subsequent cancellation for failing to disclose material information. Either way, it's not worth taking a chance. Best case scenario? Claim is paid and substantial warning issued.
When in doubt? Find out.