Good for you for not doing this! You never make mention of a gift on an invitation because you are inviting people, not presents. This is not a Canadian thing, though in some cultures it is more common to give cash instead of a gift. However, if this is the case there is no need to prompt the action. Asking for any type of gift is never polite and while I can't speak for others, I always decline invitations with gift instructions. I find it very rude as it's obvious the couple wants more than my company. If you would like cash have it spread by word of mouth when guests ask, otherwise just accept graciously the gifts that are given with love.
To answer your question, though, I did receive mostly cash gifts. But there was no formality to it, people just gave us cards when we had a moment together.