Dental work is an eligible medical expense providing the treatment wasn't purely cosmetic. Like any other medical expense, you should keep a copy of the paid bill. Because the treatment was outside Canada, it's also a good idea to keep the contact information for the dental office in case you ever need to provide proof of "necessity" rather than cosmetic reasons.

If you're filing your return via NETFILE, there's no need to submit your receipts but be sure to keep them on hand in case CRA ever requests a copy.